Section 10: Hardware Design

HARDWARE DESIGN TECHNIQUES
SECTION 10
HARDWARE DESIGN TECHNIQUES
■
Low Voltage Interfaces
■
Grounding in Mixed Signal Systems
■
Digital Isolation Techniques
■
Power Supply Noise Reduction and Filtering
■
Dealing with High Speed Logic
10.a
HARDWARE DESIGN TECHNIQUES
10.b
HARDWARE DESIGN TECHNIQUES
SECTION 10
HARDWARE DESIGN TECHNIQUES
Walt Kester
LOW VOLTAGE INTERFACES
Ethan Bordeaux, Johannes Horvath, Walt Kester
For the past 30 years, the standard VDD for digital circuits has been 5V. This voltage
level was used because bipolar transistor technology required 5V to allow headroom
for proper operation. However, in the late 1980s, Complimentary Metal Oxide
Semiconductor (CMOS) became the standard for digital IC design. This process did
not necessarily require the same voltage levels as TTL circuits, but the industry
adopted the 5V TTL standard logic threshold levels to maintain backward
compatibility with older systems (Reference 1).
The current revolution in supply voltage reduction has been driven by demand for
faster and smaller products at lower costs. This push has caused silicon geometries
to drop from 2µm in the early 1980s to 0.25µm that is used in today’s latest
microprocessor and IC designs. As feature sizes have become increasingly smaller,
the voltage for optimum device performance has also dropped below the 5V level.
This is illustrated in the current microprocessors for PCs, where the optimum core
operating voltage is programmed externally using voltage identification (VID) pins,
and can be as low as 1.3V.
The strong interest in lower voltage DSPs is clearly visible in the shifting sales
percentages for 5V and 3.3V parts. Sales growth for 3.3V DSPs has increased at
more than twice the rate of the rest of the DSP market (30% for all DSPs versus
more than 70% for 3.3V devices). This trend will continue as the high volume/high
growth portable markets demand signal processors that contain all of the traits of
the lower voltage DSPs.
On the one hand, the lower voltage ICs operate at lower power, allow smaller chip
areas, and higher speeds. On the other hand, the lower voltage ICs must often
interface to other ICs which operate at larger VDD supply voltages thereby causing
interface compatibility problems. Although lower operating voltages mean smaller
signal swings, and hence less switching noise, noise margins are lower for low
supply voltage ICs.
The popularity of 2.5V devices can be partially explained by their ability to operate
from two AA alkaline cells. Figure 10.2 shows the typical discharge characteristics
for a AA cell under various load conditions (Reference 2). Note that at a load current
of 15mA, the voltage remains above +1.25V (2.5V for two cells in series) for nearly
100 hours. Therefore, an IC that can operate effectively at low currents with a
supply voltage of 2.5V ±10% (2.25V - 2.75V) is very useful in portable designs. Also,
DSPs that have low mA/MIPS ratings and can integrate peripherals onto a single
chip, such as the ADSP-218x L or M-series, are useful in portable applications.
10.1
HARDWARE DESIGN TECHNIQUES
LOW VOLTAGE MIXED-SIGNAL ICs
■ Lower Power for Portable Applications
■ 2.5V ICs Can Operate on Two “AA” Alkaline Cells
■ Faster CMOS Processes, Smaller Geometries, Lower
Breakdown Voltages
■ Multiple Voltages in System: +5V, +3.3V, +2.5V, +1.8V
DSP Core Voltage (VID), Analog Supply Voltage
■ Interfaces Required Between Multiple Logic Types
■ Lower Voltage Swings Produce Less Switching Noise
■ Lower Noise Margins
■ Less Headroom in Analog Circuits Decreases Signal Swings
and Increases Sensitivity to Noise (But that’s the subject of an
entire seminar!)
Figure 10.1
DURACELL MN1500 “AA” ALKALINE BATTERY
DISCHARGE CHARACTERISTICS
VOLTAGE
(V)
1.25
SERVICE HOURS
Courtesy: Duracell, Inc., Berkshire Corporate Park, Bethel, CT 06801
http://www.duracell.com
Figure 10.2
10.2
HARDWARE DESIGN TECHNIQUES
In order to understand the compatibility issues relating to interfacing ICs operated
at different VDD supplies, it is useful to first look at the structure of a typical CMOS
logic stage as shown in Figure 10.3.
TYPICAL CMOS IC OUTPUT DRIVER CONFIGURATION
VDD
VDD
PMOS
High = “1”
VIH MIN
INPUT
VDD
VDD
High = “1”
VOH MIN
PREDRIVER
LOGIC
OUTPUT
NMOS
VIL MAX
Low = “0”
Low = “0”
0V
VOL MAX
0V
VIL MAX = Maximum Allowable Input Low Logic Level
VIH MIN = Minimum Allowable Input High Logic Level
VOL MAX = Maximum Allowable Output Low Logic Level
VOH MIN = Minimum Allowable Output High Logic Level
Figure 10.3
Note that the output driver stage consists of a PMOS and an NMOS transistor.
When the output is high, the PMOS transistor connects the output to the +VDD
supply through its low on-resistance (RON), and the NMOS transistor is off. When
the output is low, the NMOS transistor connects the output to ground through its
on-resistance, and the PMOS transistor is off. The RON of a CMOS output stage can
vary between 5Ω and 50Ω depending on the size of the transistors, which in turn,
determines the output current drive capability.
A typical logic IC has its power supplies and grounds separated between the output
drivers and the rest of the circuitry (including the pre-driver). This is done to
maintain a clean power supply, which reduces the effect of noise and ground bounce
on the I/O levels. This is increasingly important, since added tolerance and
compliance are critical in I/O driver specifications, especially at low voltages.
Figure 10.3 also shows “bars” which define the minimum and maximum required
input and output voltages to produce a valid high or low logic level. Note that for
CMOS logic, the actual output logic levels are determined by the drive current and
the RON of the transistors. For light loads, the output logic levels are very close to 0V
and +VDD. The input logic thresholds, on the other hand, are determined by the
input circuit of the IC.
There are three sections in the “input” bar. The bottom section shows the input
range that is interpreted as a logic low. In the case of 5V TTL, this range would be
10.3
HARDWARE DESIGN TECHNIQUES
between 0V and 0.8V. The middle section shows the input voltage range where it is
interpreted as neither a logic low nor a logic high. The upper section shows where
an input is interpreted as a logic high. In the case of 5V TTL, this would be between
2V and 5V.
Similarly, there are three sections in the “output” bar. The bottom range shows the
allowable voltage for a logic low output. In the case of 5V TTL, the IC must output a
voltage between 0V and 0.4V. The middle section shows the voltage range that is
not a valid high or low - the device should never transmit a voltage level in this
region except when transitioning from one level to the other. The upper section
shows the allowable voltage range for a logic high output signal. For 5V TTL, this
voltage is between 2.4V and 5V. The chart does not reflect a 10%
overshoot/undershoot also allowed on the inputs of the logic standard.
A summary of the existing logic standards using these definitions is shown in
Figure 10.4. Note that the input thresholds of classic CMOS logic (series-4000, for
example) are defined as 0.3VDD and 0.7VDD. However, most CMOS logic circuits in
use today are compatible with TTL and LVTTL levels which are the dominant 5V
and 3.3V operating standards for DSPs. Note that 5V TTL and 3.3V LVTTL input
and output threshold voltages are identical. The difference is the upper range for
the allowable high levels.
LOW VOLTAGE LOGIC LEVEL STANDARDS
VOLTS
5.0
4.5
4.0
5V CMOS
5V TTL
I = INPUT
I
O
I
O
O = OUTPUT
3.3V LVTTL
3.5
3.0
2.5V VCX
2.5V JEDEC
O
I
I
O
I
O
2.5V VCX output
(-12mA)
2.5V JEDEC input
2.5V JEDEC output
(-1mA)
2.0
2.5V VCX input
2.5
1.5
1.0
3.3V LVTTL output
(-2mA)
3.3V LVTTL input
5V TTL output
(-2mA)
5V TTL input
5V CMOS output
(20uA)
0
5V CMOS input
0.5
Figure 10.4
10.4
HARDWARE DESIGN TECHNIQUES
The international standards bureau JEDEC (Joint Electron Device Engineering
Council) has created a 2.5V standard (JEDEC standard 8-5) which will most likely
become the minimum requirement for 2.5V operation (Reference 3). However, there
is no current (2000) dominant 2.5V standard for IC transmission and reception,
because few manufacturers are making products that operate at this voltage. There
is one proposed 2.5V standard created by a consortium of IC manufacturers, titled
the Low Voltage Logic Alliance. Their specification provides a guideline for
semiconductor operation between 1.8V and 3.6V. A standard covering this voltage
range is useful because it ensures present and future compatibility. As an example,
the 74VCX164245, a bus translator/transceiver from Fairchild Semiconductor, is
designed to be operated anywhere between 1.8V - 3.6V and has different input and
output characteristics depending upon the supplied VDD. This standard, named
VCX, was formed by Motorola, Toshiba and Fairchild Semiconductor. It currently
consists primarily of bus transceivers, translators, FIFOs and other building block
logic. There are also a wide range of other low voltage standards, such as GTL
(Gunning Transceiver Logic), BTL (Backplane Transceiver Logic), and PECL
(PseudoECL Logic). However, most of these standards are aimed at application
specific markets and not for general purpose semiconductor systems.
The VCX devices can be operated on a very wide range of voltage levels (1.8V 3.6V). The I/O characteristics of this standard are dependent upon the VDD voltage
and the load on each pin. In Figure 10.4, one voltage (2.5V) was chosen to show the
general I/O behavior of a VCX device. Each of the device’s output voltages is listed
for a specific current. As the current requirements increase, the output high voltage
decreases while the output low voltage increases. Please refer to the appropriate
data sheets for more specific I/O information.
From this chart, it is possible to visualize some of the possible problems in
connecting together two ICs operating on different standards. One example would
be connecting a 5V CMOS device to a 3.3V LVTTL IC. The 5V CMOS high level is
too high for the LVTTL to handle ( > 3.3V). This could cause permanent damage to
the LVTTL chip. Another possible problem would be a system with a 2.5V JEDEC
IC driving a 5V CMOS device. The logic high level from the 2.5V device is not high
enough for it to register as a logic high on the 5V CMOS input (VIH MIN = 3.5V).
These examples illustrate two possible types of logic level incompatibilities - either
a device being driven with too high a voltage or a device not driving a voltage high
enough for it to register a valid high logic level with the receiving IC. These
interfacing problems introduce two important concepts: voltage tolerance and
voltage compliance.
VOLTAGE TOLERANCE AND VOLTAGE COMPLIANCE
A device that is voltage tolerant can withstand a voltage greater than its VDD on its
I/O pins. For example, if a device has a VDD of 2.5V and can accept inputs equal to
3.3V and can withstand 3.3V on its outputs, the 2.5V device is called 3.3V tolerant.
The meaning of input voltage tolerance is fairly obvious, but the meaning of output
voltage tolerance requires some explanation. The output of a 2.5V CMOS driver in
the high state appears like a small resistor (RON of the PMOS FET) connected to
2.5V. Obviously, connecting its output directly to 3.3V is likely to destroy the device
due to excessive current. However, if the 2.5V device has a three-state output which
is connected to a bus which is also driven by a 3.3V IC, then the meaning becomes
10.5
HARDWARE DESIGN TECHNIQUES
clearer. Even though the 2.5V IC is in the off (third-state) condition, the 3.3V IC can
drive the bus voltage higher than 2.5V, potentially causing damage to the 2.5V IC
output.
A device which is voltage compliant can receive signals from and transmit signals to
a device which is operated at a voltage greater than its own VDD. For example, if a
device has a 2.5V VDD and can transmit and receive signals to and from a 3.3V
device, the 2.5V device is said to be 3.3V compliant.
The interface between the 5V CMOS and 3.3V LVTTL parts illustrates a lack of
voltage tolerance; the LVTTL IC input is overdriven by the 5V CMOS device output.
The interface between the 2.5V JEDEC and the 5V CMOS part demonstrates a lack
of voltage compliance; the output high level of the JEDEC IC does not comply to the
input level requirement of a the 5V CMOS device.
LOGIC VOLTAGE TOLERANCE
AND COMPATIBILITY DEFINITIONS
■ Voltage Tolerance:
◆ A device that is Voltage Tolerant can withstand a voltage
greater than its VDD on its input and output pins. If a device
has a VDD of 2.5V and can accept inputs of 3.3V (±10%), the
2.5V device is 3.3V tolerant on its input. Input and output
tolerance should be examined and specified separately.
■ Voltage Compliance:
◆ A device that is Voltage Compliant can transmit and receive
signals to and from logic which is operated at a voltage
greater than its own VDD. If a device has a 2.5V VDD and can
properly transmit signals to and from 3.3V logic, the 2.5V
device is 3.3V compliant. Input and output compliance should
be examined and specified separately.
Figure 10.5
INTERFACING 5V TO 3.3V SYSTEMS USING NMOS FET BUS
SWITCHES
When combining ICs that operate on different voltage standards, one is often forced
to add additional discrete elements to ensure voltage tolerance and compliance. In
order to achieve voltage tolerance between 5V and 3.3V logic, for instance, a bus
switch voltage translator, or QuickSwitch™ can be used (Reference 4, 5). The bus
switch limits the voltage applied to an IC. This is done to avoid applying a larger
input high voltage than the receiving device can tolerate.
As an example, it is possible to place a bus switch between a 5V CMOS and 3.3V
LVTTL IC, and the two devices can then transmit data properly as shown in Figure
10.6. The bus switch is basically an NMOS FET. If 4.3V is placed on the gate of the
FET, the maximum passable signal is 3.3V (approximately 1V less than the gate
10.6
HARDWARE DESIGN TECHNIQUES
voltage). If both input and output are below 3.3V, the NMOS FET acts as a low
resistance (RON ≈ 5Ω). As the input approaches 3.3V, the FET on-resistance
increases, thereby limiting the signal output. The QuickSwitch contains 10 bidirectional FETs with a gate drive enable as shown in Figure 10.6. The VCC of the
QuickSwitch sets the high level for the gate drive.
One way of creating a 4.3V supply on a 5V/3.3V system board is to place a diode
between the 5V supply and VCC on the QuickSwitch. In Figure 10.6, the 4.3V is
generated by a silicon diode in series with a Schottky diode connected to the 3.3V
supply. With 10% tolerances on both 5V and 3.3V supplies, this method produces a
more stable gate bias voltage. Some bus switches are designed to operate on either
3.3V or 5V directly and generate the internal gate bias level internally.
A QuickSwitch removes voltage tolerance concerns in this mixed logic design. One
convenient feature of bus switches is that they are bi-directional; this allows the
designer to place a bus translator between two ICs and not have to create additional
routing logic for input and output signals.
+3.3V / +5V BIDIRECTIONAL INTERFACE USING
NMOS FET ACHIEVES VOLTAGE TOLERANCE
+ 3.3V BUS
≈ 1V
+ 4.3V
470 kΩ
Ω
+ 5V BUS
Si SCHOTTKY
0.1µF
3.3V ± 10%
LOGIC
NMOS
5V ± 10%
LOGIC
RON = 5 Ω
“1” = DISABLE
“0” = ENABLE
0V / VCC
QS3384 QuickSwitch ®
10 SWITCHES/PACKAGE
OPERATE WITH QS3384 VCC = +4.3V
Figure 10.6
A bus switch increases the total power dissipation along with the total area
required to layout a system. Since voltage bus switches are typically CMOS circuits,
they have very low power dissipation ratings. An average value for added
continuous power dissipation is 5mW per package (10 switches), and this is
independent of the frequency of signals which pass through the circuit. Bus
switches typically have 8 - 20 I/O pins per package and take up approximately 25 to
50mm2 of board space.
10.7
HARDWARE DESIGN TECHNIQUES
One concern when adding interface logic into a circuit is a possible increase in
propagation delay. Added propagation delay can create many timing problems in a
design. QuickSwitches have very low propagation delay values (< 0.25ns) as shown
in Figure 10.7.
QS3384 QuickSwitch® TRANSIENT RESPONSE
WITH +4.3V SUPPLY
VIN
VOUT
(VOLTS)
2.0V
0.8V
5V TTL,
3.3V LVTTL
INPUT THRESHOLDS
TIME (ns)
Courtesy: Integrated Device Technology (IDT), Inc., 2975 Stender Way, Santa Clara, CA 95054
http://www.idt.com
Figure 10.7
INTERNALLY CREATED VOLTAGE TOLERANCE / COMPLIANCE
The requirement for low power, high performance ICs has triggered a race among
manufacturers to design devices operating at and below 2.5V that are also
TTL/CMOS compatible. Figure 10.8 is a block diagram of a logic circuit that allows
the logic core to operate at a reduced voltage, while the output driver operates at a
standard supply voltage level of 3.3V, for example.
The technique followed by many IC manufacturers is to provide a secondary I/O
ring, i.e., the I/O drivers in a 2.5V IC are driven by a 3.3V power supply, hence the
device can be TTL compatible and meet the specification for VOH and VOL. The 3.3V
external power supply is required for the part to be 3.3V tolerant. This causes the
added complexity of two power supplies for the chip which have to be maintained in
all future plug-in generations of the IC.
10.8
HARDWARE DESIGN TECHNIQUES
A more flexible technique (used in the ADSP-218xM series DSPs) is to provide a
separate I/O ring with an external voltage with the option of setting that voltage
equal to the core’s operating voltage, if desired. This design can provide tolerance to
3.3V with the external voltage set to 2.5V, or 3.3V tolerance and compliance to 3.3V
with the external voltage set to 3.3V. There are vendors today that use this option
partially, i.e., the VCX devices are 3.3V tolerant at 2.5V internal and external
voltages, but do not have the option of 3.3V compliance. Other existing designs and
patents that address this issue do not support complete tolerance and compliance
and the low standby current specification. This approach is complicated, since the
circuits must meet the noise and power requirements with the external voltages at
3.3V or 2.5V.
CMOS IC WITH SECONDARY I/O RING
VDD INTERNAL(CORE)
VDD EXTERNAL
SECONDARY
I/O RING
PMOS
CORE
LOGIC
INPUT
OUTPUT
NMOS
INTERNAL GND
ADSP-21xx M-SERIES
EXTERNAL GND
VDD EXTERNAL = +3.3V OR +2.5V
= +2.5V
VDD CORE
INPUT +3.3V TOLERANT
Figure 10.8
There are several issues to consider in a dual-supply logic IC design:
•
Power-Up Sequencing: If two power supplies are required to give an IC
additional tolerance / compliance, what is the power-up sequence? Is it a
requirement that the power supplies are switched on simultaneously or can the
device only have a voltage supplied on the core or only on the I/O ring?
10.9
HARDWARE DESIGN TECHNIQUES
•
Process Support and Electro-Static Discharge (ESD) Protection: The transistors
created in the IC’s fabrication process must be able to both withstand and drive
high voltages. The high voltage transistors create additional fabrication costs
since they require more processing steps to build in high voltage tolerance.
Designs with standard transistors require additional circuitry. The I/O drivers
must also provide ESD protection for the device. Most current designs limit the
overvoltage to below one diode drop (0.7V) above the power supply. Protection
for larger overvoltage requires more diodes in series.
•
Internal High Voltage Generation: The PMOS transistors need to be placed in a
substrate well which is tied to the highest on-chip voltage to prevent lateral
diodes from turning on and drawing excessive current. This high voltage can
either be generated on chip using charge pumps or from an external supply. This
requirement can make the design complex, since one cannot efficiently use
charge pumps to generate higher voltages and also achieve low standby current.
•
Chip Area: Die size is a primary factor in reducing costs and increasing yields.
Tolerance and compliance circuitry may require either more or larger I/O devices
to achieve the desired performance levels.
•
Testing: Since the core and the I/O can be at different voltages, testing the
device for all possible combinations of voltages can be complicated, adding to the
total cost of the IC.
3.3V / 2.5V INTERFACES
The Fairchild 74VCX164245-series are low voltage 16-bit dual supply logic
translators/transceivers with three-state outputs. A simplified block diagram is
shown in Figure 10.9. These devices use the VCX low voltage standard previously
discussed. The output driver circuit is supplied from the VDDB power supply bus,
ensuring VDDB compliant and tolerant outputs. The input circuit is supplied from the
VDDA supply, and the input logic threshold adjust circuits optimize the input logic
thresholds for the particular value of VDDA. Figure 10.10 shows the VCX voltage
standards for 3.3V, 2.5V, and 1.8V supply voltages. Note that the input voltage is
3.3V tolerant for all three supply voltages.
These devices dissipate about 2mW per input/output and are packaged in a 48-lead
TSSOP with a 2.5V supply. Propagation delay is about 3.2ns.
Figure 10.11 shows two possibilities for a 3.3V to 2.5V logic interface. The top
diagram (A) shows a direct connection. This will work provided the 2.5V IC is 3.3V
tolerant on its input. If the 2.5V IC is not 3.3V tolerant, the VCX translator can be
used as shown in Figure 10.11B.
10.10
HARDWARE DESIGN TECHNIQUES
LOGIC TRANSLATING TRANSCEIVER
(FAIRCHILD 74VCX164245)
VDDA BUS
VDDB BUS
VDDA
INPUT
LOGIC
INPUT
LOGIC
THRESHOLD
ADJUST
CIRCUITS
VDDB
PREDRIVER
LOGIC
PMOS
OUTPUT
LOGIC
NMOS
74VCX164245 CHARACTERISTICS:
■ Power Dissipation = 2mW / Input or Output
■ 16-bits per 48-lead TSSOP 100mm2 package
■ 3.2ns Propagation Delay at +2.5V
Figure 10.9
VOLTAGE COMPLIANCE FOR VCX STANDARD
(FAIRCHILD 74VCX164245 TRANSLATOR)
3.3V
3.5
3.0
I
1.8V
2.5V
O
I = INPUT
VOLTS
O = OUTPUT
2.5
I
O
2.0
I
1.5
O
1.0
0.5
0
–12mA
–12mA
–6mA
Figure 10.10
10.11
HARDWARE DESIGN TECHNIQUES
+3.3V TO +2.5V INTERFACE
+3.3V BUS
A
+3.3V ± 10%
LOGIC
+2.5V BUS
DIRECT
CONNECTION
+3.3V BUS
+2.5V ± 10%
LOGIC
OK IF 2.5V LOGIC
IS 3.3V TOLERANT
(3.6V MAX.)
+2.5V BUS
TRANSLATOR
B
+3.3V ± 10%
LOGIC
+2.5V ± 10%
LOGIC
Figure 10.11
Figure 10.12A shows a direct connection between 2.5V and 3.3V logic. In order for
this to work, the 2.5V output must be at least 2V minimum. With no loading on the
2.5V output, the 3.3V IC input is connected directly to +2.5V through the onresistance of the PMOS transistor driver. This provides 0.5V noise margin for the
nominal supply voltage of 2.5V. However, the 10% tolerance on the 2.5V bus allows
it to drop to a minimum of 2.25V, and the noise margin is reduced to 0.25V. This
may still work in a relatively quiet environment, but could be marginal if there is
noise on the supply voltages.
Adding a 1.6kΩ pull-up resistor as shown in Figure 10.12B ensures the 2.5V output
will not drop below 2.5V due to the input current of the 3.3V device, but the
degraded noise margin still exists for a 2.25V supply. With a 50% duty cycle, the
resistor adds about 3.4mW power dissipation per output.
A more reliable interface between 2.5V and 3.3V logic is shown in Figure 10.12C,
where a VCX translator is used. This solves all noise margin problems associated
with (A) and (B) and requires about 2mW per output.
10.12
HARDWARE DESIGN TECHNIQUES
+2.5V TO +3.3V INTERFACE
+2.5V BUS
A
+2.5V ± 10%
LOGIC
(+2.25V MIN.)
+2.5V BUS
B
+2.5V ± 10%
LOGIC
(+2.25V MIN.)
+3.3V BUS
DIRECT
CONNECTION
+3.3V ± 10%
LOGIC
NEEDS
+2V MIN.
PULL UP
RESISTOR
1.6kΩ
Ω
3.4mW
50% D.C.
+3.3V BUS
+3.3V ± 10%
LOGIC
TRANSLATOR
+2.5V BUS
C
+2.5V ± 10%
LOGIC
(+2.25V MIN.)
+3.3V BUS
+3.3V ± 10%
LOGIC
2mW
Figure 10.12
10.13
HARDWARE DESIGN TECHNIQUES
REFERENCES ON LOW VOLTAGE INTERFACES
1.
P. Alfke, Low-Voltage FPGAs Allow 3.3V/5V System Design,
Electronic Design, p. 70-76, August 18, 1997.
2.
AA Alkaline Battery Discharge Characteristics, Duracell Inc.,
Berkshire Corporate Park, Bethel, CT 06801, http://www.duracell.com
3.
Joint Electron Device Engineering Council (JEDEC),
Standard 8-5, October 1995.
4.
QS3384 Data Sheet, Integrated Device Technology (IDT), Inc.,
2975 Stender Way, Santa Clara, CA 95054, http://www.idt.com
5.
Pericom Semiconductor Corporation, 2380 Bering Drive,
San Jose, CA 95131, http://www.pericom.com
6.
74VCX164245 Data Sheet, Fairchild Semiconductor, 1997.
http://www.fairchildsemi.com
7.
H. Johnson, M. Graham, High Speed Digital Design, Prentice Hall, 1993.
10.14
HARDWARE DESIGN TECHNIQUES
GROUNDING IN MIXED SIGNAL SYSTEMS
Walt Kester, James Bryant, Mike Byrne
Today's signal processing systems generally require mixed-signal devices such as
analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) as well
as fast digital signal processors (DSPs). Requirements for processing analog signals
having wide dynamic ranges increases the importance of high performance ADCs
and DACs. Maintaining wide dynamic range with low noise in hostile digital
environments is dependent upon using good high-speed circuit design techniques
including proper signal routing, decoupling, and grounding.
In the past, "high precision, low-speed" circuits have generally been viewed
differently than so-called "high-speed" circuits. With respect to ADCs and DACs, the
sampling (or update) frequency has generally been used as the distinguishing speed
criteria. However, the following two examples show that in practice, most of today's
signal processing ICs are really "high-speed," and must therefore be treated as such
in order to maintain high performance. This is certainly true of DSPs, and also true
of ADCs and DACs.
All sampling ADCs (ADCs with an internal sample-and-hold circuit) suitable for
signal processing applications operate with relatively high speed clocks with fast
rise and fall times (generally a few nanoseconds) and must be treated as high speed
devices, even though throughput rates may appear low. For example, the 12-bit
AD7892 successive approximation (SAR) ADC operates on an 8MHz internal clock,
while the sampling rate is only 600kSPS.
Sigma-delta (Σ-∆) ADCs also require high speed clocks because of their high
oversampling ratios. The AD7722 16-bit ADC has an output data rate (effective
sampling rate) of 195kSPS, but actually samples the input signal at 12.5MSPS (64times oversampling). Even high resolution, so-called "low frequency" Σ-∆ industrial
measurement ADCs (having throughputs of 10Hz to 7.5kHz) operate on 5MHz or
higher clocks and offer resolution to 24-bits (for example, the Analog Devices
AD7730 and AD7731).
To further complicate the issue, mixed-signal ICs have both analog and digital
ports, and because of this, much confusion has resulted with respect to proper
grounding techniques. In addition, some mixed-signal ICs have relatively low digital
currents, while others have high digital currents. In many cases, these two types
must be treated differently with respect to optimum grounding.
Digital and analog design engineers tend to view mixed-signal devices from
different perspectives, and the purpose of this section is to develop a general
grounding philosophy that will work for most mixed signal devices, without having
to know the specific details of their internal circuits.
GROUND AND POWER PLANES
The importance of maintaining a low impedance large area ground plane is critical
to all analog circuits today. The ground plane not only acts as a low impedance
return path for decoupling high frequency currents (caused by fast digital logic) but
10.15
HARDWARE DESIGN TECHNIQUES
also minimizes EMI/RFI emissions. Because of the shielding action of the ground
plane, the circuit’s susceptibility to external EMI/RFI is also reduced.
Ground planes also allow the transmission of high speed digital or analog signals
using transmission line techniques (microstrip or stripline) where controlled
impedances are required.
The use of "buss wire" is totally unacceptable as a "ground" because of its
impedance at the equivalent frequency of most logic transitions. For instance, #22
gauge wire has about 20nH/inch inductance. A transient current having a slew rate
of 10mA/ns created by a logic signal would develop an unwanted voltage drop of
200mV at this frequency flowing through 1 inch of this wire:
∆v = L
10mA
∆i
= 20 nH ×
= 200mV.
ns
∆t
For a signal having a 2V peak-to-peak range, this translates into an error of about
200mV, or 10% (approximate 3.5-bit accuracy). Even in all-digital circuits, this error
would result in considerable degradation of logic noise margins.
Figure 10.13 shows an illustration of a situation where the digital return current
modulates the analog return current (top figure). The ground return wire
inductance and resistance is shared between the analog and digital circuits, and
this is what causes the interaction and resulting error. A possible solution is to
make the digital return current path flow directly to the GND REF as shown in the
bottom figure. This is the fundamental concept of a "star," or single-point ground
system. Implementing the true single-point ground in a system which contains
multiple high frequency return paths is difficult because the physical length of the
individual return current wires will introduce parasitic resistance and inductance
which can make obtaining a low impedance high frequency ground difficult. In
practice, the current returns must consist of large area ground planes for low
impedance to high frequency currents. Without a low-impedance ground plane, it is
therefore almost impossible to avoid these shared impedances, especially at high
frequencies.
All integrated circuit ground pins should be soldered directly to the low-impedance
ground plane to minimize series inductance and resistance. The use of traditional
IC sockets is not recommended with high-speed devices. The extra inductance and
capacitance of even "low profile" sockets may corrupt the device performance by
introducing unwanted shared paths. If sockets must be used with DIP packages, as
in prototyping, individual "pin sockets" or "cage jacks" may be acceptable. Both
capped and uncapped versions of these pin sockets are available (AMP part
numbers 5-330808-3, and 5-330808-6). They have spring-loaded gold contacts which
make good electrical and mechanical connection to the IC pins. Multiple insertions,
however, may degrade their performance.
Power supply pins should be decoupled directly to the ground plane using low
inductance ceramic surface mount capacitors. If through-hole mounted ceramic
capacitors must be used, their leads should be less than 1mm. The ceramic
capacitors should be located as close as possible to the IC power pins. Ferrite beads
may be also required for additional decoupling.
10.16
HARDWARE DESIGN TECHNIQUES
DIGITAL CURRENTS FLOWING IN ANALOG
RETURN PATH CREATE ERROR VOLTAGES
ID
IA
+
VD
INCORRECT
+
VA
ANALOG
CIRCUITS
VIN
GND
REF
IA + ID
DIGITAL
CIRCUITS
ID
ID
IA
+
VD
+
VA
GND
REF
VIN
CORRECT
ANALOG
CIRCUITS
DIGITAL
CIRCUITS
IA
ID
Figure 10.13
DOUBLE-SIDED VS. MULTILAYER PRINTED CIRCUIT BOARDS
Each PCB in the system should have at least one complete layer dedicated to the
ground plane. Ideally, a double-sided board should have one side completely
dedicated to ground and the other side for interconnections. In practice, this is not
possible, since some of the ground plane will certainly have to be removed to allow
for signal and power crossovers, vias, and through-holes. Nevertheless, as much
area as possible should be preserved, and at least 75% should remain. After
completing an initial layout, the ground layer should be checked carefully to make
sure there are no isolated ground "islands," because IC ground pins located in a
ground "island" have no current return path to the ground plane. Also, the ground
plane should be checked for "skinny" connections between adjacent large areas
which may significantly reduce the effectiveness of the ground plane. Needless to
say, auto-routing board layout techniques will generally lead to a layout disaster on
a mixed-signal board, so manual intervention is highly recommended.
Systems that are densely packed with surface mount ICs will have a large number
of interconnections; therefore multilayer boards are mandatory. This allows at least
one complete layer to be dedicated to ground. A simple 4-layer board would have
internal ground and power plane layers with the outer two layers used for
10.17
HARDWARE DESIGN TECHNIQUES
interconnections between the surface mount components. Placing the power and
ground planes adjacent to each other provides additional inter-plane capacitance
which helps high frequency decoupling of the power supply. In most systems, 4layers are not enough, and additional layers are required for routing signals as well
as power.
GROUND PLANES ARE MANDATORY!
■ Use Large Area Ground (and Power) Planes for Low Impedance
Current Return Paths (Must Use at Least a Double-Sided Board!)
■ Double-Sided Boards:
◆ Avoid High-Density Interconnection Crossovers and Vias
Which Reduce Ground Plane Area
◆ Keep > 75% Board Area on One Side for Ground Plane
■ Multilayer Boards: Mandatory for Dense Systems
◆ Dedicate at Least One Layer for the Ground Plane
◆ Dedicate at Least One Layer for the Power Plane
■ Use at Least 30% to 40% of PCB Connector Pins for Ground
■ Continue the Ground Plane on the Backplane Motherboard to
Power Supply Return
Figure 10.14
MULTICARD MIXED-SIGNAL SYSTEMS
The best way of minimizing ground impedance in a multicard system is to use a
"motherboard" PCB as a backplane for interconnections between cards, thus
providing a continuous ground plane to the backplane. The PCB connector should
have at least 30-40% of its pins devoted to ground, and these pins should be
connected to the ground plane on the backplane mother card. To complete the
overall system grounding scheme there are two possibilities:
1. The backplane ground plane can be connected to chassis ground at numerous
points, thereby diffusing the various ground current return paths. This is commonly
referred to as a "multipoint" grounding system and is shown in Figure 10.15.
2. The ground plane can be connected to a single system "star ground" point
(generally at the power supply).
10.18
HARDWARE DESIGN TECHNIQUES
MULTIPOINT GROUND CONCEPT
VA
PCB
VD
VA
GROUND PLANE
PCB
VD
GROUND PLANE
BACKPLANE
GROUND PLANE
CHASSIS
GROUND
POWER
SUPPLIES
VA
VD
Figure 10.15
The first approach is most often used in all-digital systems, but can be used in
mixed-signal systems provided the ground currents due to digital circuits are
sufficiently low and diffused over a large area. The low ground impedance is
maintained all the way through the PC boards, the backplane, and ultimately the
chassis. However, it is critical that good electrical contact be made where the
grounds are connected to the sheet metal chassis. This requires self-tapping sheet
metal screws or "biting" washers. Special care must be taken where anodized
aluminum is used for the chassis material, since its surface acts as an insulator.
The second approach ("star ground") is often used in high speed mixed-signal
systems having separate analog and digital ground systems and warrants further
discussion.
SEPARATING ANALOG AND DIGITAL GROUNDS
In mixed-signal systems with large amounts of digital circuitry, it is highly
desirable to physically separate sensitive analog components from noisy digital
components. It may also be beneficial to use separate ground planes for the analog
and the digital circuitry. These planes should not overlap in order to minimize
capacitive coupling between the two. The separate analog and digital ground planes
are continued on the backplane using either motherboard ground planes or "ground
screens" which are made up of a series of wired interconnections between the
10.19
HARDWARE DESIGN TECHNIQUES
connector ground pins. The arrangement shown in Figure 10.16 illustrates that the
two planes are kept separate all the way back to a common system "star" ground,
generally located at the power supplies. The connections between the ground planes,
the power supplies, and the "star" should be made up of multiple bus bars or wide
copper braids for minimum resistance and inductance. The back-to-back Schottky
diodes on each PCB are inserted to prevent accidental DC voltage from developing
between the two ground systems when cards are plugged and unplugged. This
voltage should be kept less than 300mV to prevent damage to ICs which have
connections to both the analog and digital ground planes. Schottky diodes are
preferable because of their low capacitance and low forward voltage drop. The low
capacitance prevents AC coupling between the analog and digital ground planes.
Schottky diodes begin to conduct at about 300mV, and several parallel diodes in
parallel may be required if high currents are expected. In some cases, ferrite beads
can be used instead of Schottky diodes, however they introduce DC ground loops
which can be troublesome in precision systems.
SEPARATING ANALOG AND DIGITAL GROUND PLANES
VA
ANALOG
GROUND
PLANE
PCB
VD
VA
DIGITAL
GROUND
PLANE
ANALOG
GROUND
PLANE
D
A
PCB
VD
DIGITAL
GROUND
PLANE
D
A
DIGITAL GROUND PLANE
BACKPLANE
ANALOG GROUND PLANE
POWER
SUPPLIES
SYSTEM
STAR
GROUND
VA
VD
Figure 10.16
It is mandatory that the impedance of the ground planes be kept as low as possible,
all the way back to the system star ground. DC or AC voltages of more than 300mV
between the two ground planes can not only damage ICs but cause false triggering
of logic gates and possible latchup.
10.20
HARDWARE DESIGN TECHNIQUES
GROUNDING AND DECOUPLING MIXED-SIGNAL ICS WITH LOW DIGITAL
CURRENTS
Sensitive analog components such as amplifiers and voltage references are always
referenced and decoupled to the analog ground plane. The ADCs and DACs (and
other mixed-signal ICs) with low digital currents should generally be treated as
analog components and also grounded and decoupled to the analog ground plane. At
first glance, this may seem somewhat contradictory, since a converter has an analog
and digital interface and usually has pins designated as analog ground (AGND) and
digital ground (DGND). The diagram shown in Figure 10.17 will help to explain this
seeming dilemma.
PROPER GROUNDING OF MIXED-SIGNAL ICs
WITH LOW INTERNAL DIGITAL CURRENTS
VA
VD
FERRITE BEAD
A
LP
LP
CSTRAY
RP
AIN/
OUT
ANALOG
CIRCUITS
LP
R
B
CSTRAY
IA
ID
AGND
A
SEE
TEXT
RP
DIGITAL
CIRCUITS DATA
A
RP
D
VD
VA
SHORT
CONNECTIONS
BUFFER
GATE OR
REGISTER
DATA
BUS
CIN ≈ 10pF
RP
LP
DGND
A
A = ANALOG GROUND PLANE
VNOISE
D
D = DIGITAL GROUND PLANE
Figure 10.17
10.21
HARDWARE DESIGN TECHNIQUES
Inside an IC that has both analog and digital circuits, such as an ADC or a DAC,
the grounds are usually kept separate to avoid coupling digital signals into the
analog circuits. Figure 10.17 shows a simple model of a converter. There is nothing
the IC designer can do about the wirebond inductance and resistance associated
with connecting the bond pads on the chip to the package pins except to realize it's
there. The rapidly changing digital currents produce a voltage at point B which will
inevitably couple into point A of the analog circuits through the stray capacitance,
CSTRAY. In addition, there is approximately 0.2pF unavoidable stray capacitance
between every pin of the IC package! It's the IC designer's job to make the chip
work in spite of this. However, in order to prevent further coupling, the AGND and
DGND pins should be joined together externally to the analog ground plane with
minimum lead lengths. Any extra impedance in the DGND connection will cause
more digital noise to be developed at point B; it will, in turn, couple more digital
noise into the analog circuit through the stray capacitance. Note that connecting
DGND to the digital ground plane applies VNOISE across the AGND and DGND
pins and invites disaster!
The name "DGND" on an IC tells us that this pin connects to the digital ground of
the IC. This does not imply that this pin must be connected to the digital ground of
the system.
It is true that this arrangement may inject a small amount of digital noise onto the
analog ground plane. These currents should be quite small, and can be minimized
by ensuring that the converter output does not drive a large fanout (they normally
can't, by design). Minimizing the fanout on the converter's digital port will also keep
the converter logic transitions relatively free from ringing and minimize digital
switching currents, and thereby reducing any potential coupling into the analog port
of the converter. The logic supply pin (VD) can be further isolated from the analog
supply by the insertion of a small lossy ferrite bead as shown in Figure 10.17. The
internal transient digital currents of the converter will flow in the small loop from
VD through the decoupling capacitor and to DGND (this path is shown with a heavy
line on the diagram). The transient digital currents will therefore not appear on the
external analog ground plane, but are confined to the loop. The VD pin decoupling
capacitor should be mounted as close to the converter as possible to minimize
parasitic inductance. These decoupling capacitors should be low inductance ceramic
types, typically between 0.01µF and 0.1µF.
TREAT THE ADC DIGITAL OUTPUTS WITH CARE
It is always a good idea (as shown in Figure 10.17) to place a buffer register
adjacent to the converter to isolate the converter's digital lines from noise on the
data bus. The register also serves to minimize loading on the digital outputs of the
converter and acts as a Faraday shield between the digital outputs and the data
bus. Even though many converters have three-state outputs/inputs, this isolation
register still represents good design practice. In some cases it may be desirable to
10.22
HARDWARE DESIGN TECHNIQUES
add an additional buffer register on the analog ground plane next to the converter
output to provide greater isolation.
The series resistors (labeled "R" in Figure 10.17) between the ADC output and the
buffer register input help to minimize the digital transient currents which may
affect converter performance. The resistors isolate the digital output drivers from
the capacitance of the buffer register inputs. In addition, the RC network formed by
the series resistor and the buffer register input capacitance acts as a lowpass filter
to slow down the fast edges.
A typical CMOS gate combined with PCB trace and a through-hole will create a load
of approximately 10pF. A logic output slew rate of 1V/ns will produce 10mA of
dynamic current if there is no isolation resistor:
1V
∆v
= 10pF ×
= 10mA .
ns
∆t
A 500Ω series resistors will minimize this output current and result in a rise and
fall time of approximately 11ns when driving the 10pF input capacitance of the
register:
∆I = C
t r = 2.2 × τ = 2.2 × R ⋅ C = 2.2 × 500Ω × 10 pF = 11ns.
TTL registers should be avoided, since they can appreciably add to the dynamic
switching currents because of their higher input capacitance.
The buffer register and other digital circuits should be grounded and decoupled to
the digital ground plane of the PC board. Notice that any noise between the analog
and digital ground plane reduces the noise margin at the converter digital interface.
Since digital noise immunity is of the orders of hundreds or thousands of millivolts,
this is unlikely to matter. The analog ground plane will generally not be very noisy,
but if the noise on the digital ground plane (relative to the analog ground plane)
exceeds a few hundred millivolts, then steps should be taken to reduce the digital
ground plane impedance, thereby maintaining the digital noise margins at an
acceptable level. Under no circumstances should the voltage between the two
ground planes exceed 300mV, or the ICs may be damaged.
Separate power supplies for analog and digital circuits are also highly desirable.
The analog supply should be used to power the converter. If the converter has a pin
designated as a digital supply pin (VD), it should either be powered from a separate
analog supply, or filtered as shown in the diagram. All converter power pins should
be decoupled to the analog ground plane, and all logic circuit power pins should be
decoupled to the digital ground plane as shown in Figure 10.18. If the digital power
supply is relatively quiet, it may be possible to use it to supply analog circuits as
well, but be very cautious.
In some cases it may not be possible to connect VD to the analog supply. Some of the
newer, high speed ICs may have their analog circuits powered by +5V, but the
digital interface powered by +3V to interface to 3V logic. In this case, the +3V pin of
the IC should be decoupled directly to the analog ground plane. It is also advisable
to connect a ferrite bead in series with the power trace that connects the pin to the
+3V digital logic supply.
10.23
HARDWARE DESIGN TECHNIQUES
GROUNDING AND DECOUPLING POINTS
VA FERRITE
VA
VD
BEAD
SEE
TEXT
A
A
VA
VD
D
R
ADC
OR
DAC
AMP
A
VA
AGND
A
R
A
A
SAMPLING
CLOCK
GENERATOR
A
TO OTHER
DIGITAL
CIRCUITS
DGND
A
VOLTAGE
REFERENCE
BUFFER
GATE
OR
REGISTER
D
VA
A
A
ANALOG
GROUND PLANE
D
DIGITAL
GROUND PLANE
Figure 10.18
The sampling clock generation circuitry should be treated like analog circuitry and
also be grounded and heavily-decoupled to the analog ground plane. Phase noise on
the sampling clock produces degradation in system SNR as will be discussed
shortly.
SAMPLING CLOCK CONSIDERATIONS
In a high performance sampled data system a low phase-noise crystal oscillator
should be used to generate the ADC (or DAC) sampling clock because sampling
clock jitter modulates the analog input/output signal and raises the noise and
distortion floor. The sampling clock generator should be isolated from noisy digital
circuits and grounded and decoupled to the analog ground plane, as is true for the
op amp and the ADC.
10.24
HARDWARE DESIGN TECHNIQUES
The effect of sampling clock jitter on ADC Signal-to-Noise Ratio (SNR) is given
approximately by the equation:
 1 
SNR = 20 log10 
,
 2π ft j 
where SNR is the SNR of a perfect ADC of infinite resolution where the only source
of noise is that caused by the RMS sampling clock jitter, tj. Note that f in the above
equation is the analog input frequency. Just working through a simple example, if tj
= 50ps RMS, f = 100kHz, then SNR = 90dB, equivalent to about 15-bits dynamic
range.
It should be noted that tj in the above example is the root-sum-square (RSS) value
of the external clock jitter and the internal ADC clock jitter (called aperture jitter).
However, in most high performance ADCs, the internal aperture jitter is negligible
compared to the jitter on the sampling clock.
Since degradation in SNR is primarily due to external clock jitter, steps must be
taken to ensure the sampling clock is as noise-free as possible and has the lowest
possible phase jitter. This requires that a crystal oscillator be used. There are
several manufacturers of small crystal oscillators with low jitter (less than 5ps
RMS) CMOS compatible outputs. (For example, MF Electronics, 10 Commerce Dr.,
New Rochelle, NY 10801, Tel. 914-576-6570.)
Ideally, the sampling clock crystal oscillator should be referenced to the analog
ground plane in a split-ground system. However, this is not always possible because
of system constraints. In many cases, the sampling clock must be derived from a
higher frequency multi-purpose system clock which is generated on the digital
ground plane. It must then pass from its origin on the digital ground plane to the
ADC on the analog ground plane. Ground noise between the two planes adds
directly to the clock signal and will produce excess jitter. The jitter can cause
degradation in the signal-to-noise ratio and also produce unwanted harmonics. This
can be remedied somewhat by transmitting the sampling clock signal as a
differential signal using either a small RF transformer as shown in Figure 10.19 or
a high speed differential driver and receiver IC. If an active differential driver and
receiver are used, they should be ECL to minimize phase jitter. In a single +5V
supply system, ECL logic can be connected between ground and +5V (PECL), and
the outputs AC coupled into the ADC sampling clock input. In either case, the
original master system clock must be generated from a low phase noise crystal
oscillator.
10.25
HARDWARE DESIGN TECHNIQUES
SAMPLING CLOCK DISTRIBUTION FROM
DIGITAL TO ANALOG GROUND PLANES
DIGITAL GROUND PLANE
VD
VD
LOW PHASE
NOISE
MASTER CLOCK
D
ANALOG GROUND PLANE
SAMPLING
CLOCK
SYSTEM CLOCK
GENERATORS
VD
METHOD 1
D
D
A
VD
DSP OR MICROPROCESSOR
VA
+
SAMPLING
CLOCK
_
METHOD 2
D
SNR = 20 log10
D
1
2π f tj
A
tj = Sampling Clock Jitter
f = Analog Input Frequency
Figure 10.19
THE ORIGINS OF THE CONFUSION ABOUT MIXED-SIGNAL GROUNDING:
APPLYING SINGLE-CARD GROUNDING CONCEPTS TO MULTICARD SYSTEMS
Most ADC, DAC, and other mixed-signal device data sheets discuss grounding
relative to a single PCB, usually the manufacturer's own evaluation board. This has
been a source of confusion when trying to apply these principles to multicard or
multi-ADC/DAC systems. The recommendation is usually to split the PCB ground
plane into an analog plane and a digital plane. It is then further recommended that
the AGND and DGND pins of a converter be tied together and that the analog
ground plane and digital ground planes be connected at that same point as shown in
Figure 10.20. This essentially creates the system "star" ground at the mixed-signal
device.
All noisy digital currents flow through the digital power supply to the digital ground
plane and back to the digital supply; they are isolated from the sensitive analog
portion of the board. The system star ground occurs where the analog and digital
ground planes are joined together at the mixed signal device. While this approach
will generally work in a simple system with a single PCB and single ADC/DAC, it is
not optimum for multicard mixed-signal systems. In systems having several ADCs
or DACs on different PCBs (or on the same PCB, for that matter), the analog and
digital ground planes become connected at several points, creating the possibility of
ground loops and making a single-point "star" ground system impossible. For these
reasons, this grounding approach is not recommended for multicard systems, and
10.26
HARDWARE DESIGN TECHNIQUES
the approach previously discussed should be used for mixed signal ICs with low
digital currents.
GROUNDING MIXED SIGNAL ICs : SINGLE PC BOARD
(TYPICAL EVALUATION/TEST BOARD)
VD
VA
VA
MIXED
SIGNAL
DEVICE
ANALOG
CIRCUITS
AGND
SYSTEM
STAR
GROUND
A
VD
DIGITAL
CIRCUITS
DGND
A
D
ANALOG
GROUND PLANE
D
DIGITAL
GROUND PLANE
D
A
ANALOG
SUPPLY
DIGITAL
SUPPLY
Figure 10.20
SUMMARY: GROUNDING MIXED SIGNAL DEVICES WITH LOW DIGITAL
CURRENTS IN A MULTICARD SYSTEM
Figure 10.21 summarizes the approach previously described for grounding a mixed
signal device which has low digital currents. The analog ground plane is not
corrupted because the small digital transient currents flow in the small loop
between VD, the decoupling capacitor, and DGND (shown as a heavy line). The
mixed signal device is for all intents and purposes treated as an analog component.
The noise VN between the ground planes reduces the noise margin at the digital
interface but is generally not harmful if kept less than 300mV by using a low
impedance digital ground plane all the way back to the system star ground.
However, mixed signal devices such as sigma-delta ADCs, codecs, and DSPs with
on-chip analog functions are becoming more and more digitally intensive. Along
with the additional digital circuitry come larger digital currents and noise. For
example, a sigma-delta ADC or DAC contains a complex digital filter which adds
considerably to the digital current in the device. The method previously discussed
depends on the decoupling capacitor between VD and DGND to keep the digital
transient currents and isolated in a small loop. However, if the digital currents are
10.27
HARDWARE DESIGN TECHNIQUES
significant enough and have components at DC or low frequencies, the decoupling
capacitor may have to be so large that it is impractical. Any digital current which
flows outside the loop between VD and DGND must flow through the analog ground
plane. This may degrade performance, especially in high resolution systems.
GROUNDING MIXED SIGNAL ICs WITH LOW
INTERNAL DIGITAL CURRENTS: MULTIPLE PC BOARDS
VN
VD
VA
FILTER
VA
VD
MIXED
SIGNAL
DEVICE
ANALOG
CIRCUITS
AGND
A
VN = NOISE BETWEEN
GROUND PLANES
R
BUS
BUFFER
LATCH
DGND
A
D
A
ANALOG
GROUND PLANE
A
DIGITAL
CIRCUITS
D
DIGITAL
GROUND PLANE
A
D
TO SYSTEM
ANALOG SUPPLY
D
TO SYSTEM
DIGITAL SUPPLY
TO SYSTEM STAR GROUND
Figure 10.21
It is difficult to predict what level of digital current flowing into the analog ground
plane will become unacceptable in a system. All we can do at this point is to suggest
an alternative grounding method which may yield better performance.
SUMMARY: GROUNDING MIXED SIGNAL DEVICES WITH HIGH DIGITAL
CURRENTS IN A MULTICARD SYSTEM
An alternative grounding method for a mixed signal device with high levels of
digital currents is shown in Figure 10.22. The AGND of the mixed signal device is
connected to the analog ground plane, and the DGND of the device is connected to
the digital ground plane. The digital currents are isolated from the analog ground
plane, but the noise between the two ground planes is applied directly between the
AGND and DGND pins of the device. For this method to be successful, the analog
and digital circuits within the mixed signal device must be well isolated. The noise
between AGND and DGND pins must not be large enough to reduce internal noise
margins or cause corruption of the internal analog circuits.
10.28
HARDWARE DESIGN TECHNIQUES
GROUNDING ALTERNATIVE FOR MIXED SIGNAL ICs WITH
HIGH DIGITAL CURRENTS: MULTIPLE PC BOARDS
VN = NOISE BETWEEN
GROUND PLANES
VN
VD
VA
✱
BACK-TO-BACK
SCHOTTKY
DIODES
OR
FERRITE
BEAD
(SEE TEXT)
VA
VD
MIXED
SIGNAL
DEVICE
ANALOG
CIRCUITS
AGND
A
DIGITAL
CIRCUITS
DGND
A
D
D
✱
ANALOG
GROUND PLANE
A
TO SYSTEM
ANALOG SUPPLY
A
DIGITAL
GROUND PLANE
D
TO SYSTEM STAR GROUND
D
TO SYSTEM
DIGITAL SUPPLY
Figure 10.22
Figure 10.22 shows optional Schottky diodes (back-to-back) or a ferrite bead
connecting the analog and digital ground planes. The Schottky diodes prevent large
DC voltages or low frequency voltage spikes from developing across the two planes.
These voltages can potentially damage the mixed signal IC if they exceed 300mV
because they appear directly between the AGND and DGND pins. As an alternative
to the back-to-back Schottky diodes, a ferrite bead provides a DC connection
between the two planes but isolates them at frequencies above a few MHz where the
ferrite bead becomes resistive. This protects the IC from DC voltages between
AGND and DGND, but the DC connection provided by the ferrite bead can
introduce unwanted DC ground loops and may not be suitable for high resolution
systems.
GROUNDING DSPS WITH INTERNAL PHASE-LOCKED LOOPS
As if dealing with mixed-signal ICs with AGND and DGNDs wasn’t enough, newer
DSPs such as the ADSP-21160 SHARC with internal phase-locked-loops (PLLs)
raise issues with respect to proper grounding. The ADSP-21160 PLL allows the
internal core clock (determines the instruction cycle time) to operate at a userselectable ratio of 2, 3, or 4 times the external clock frequency, CLKIN. The CLKIN
rate is the rate at which the synchronous external ports operates. Although this
allows using a lower frequency external clock, care must be taken with the power
and ground connections to the internal PLL as shown in Figure 10.23.
10.29
HARDWARE DESIGN TECHNIQUES
GROUNDING DSPs WITH INTERNAL
PHASE-LOCKED-LOOPS (PLLs)
+3.3V
10Ω
Ω
+2.5V
SHORT TRACES
40
AVDD
0.1µF
0.01µF
VDD INT
46
VDD EXT
PLL
X1, X2,
X3, X4
DSP
(ADSP-21160)
CLKIN
AGND
GND
83
SHORT TRACES
DIGITAL GROUND PLANE
Figure 10.23
In order to prevent internal coupling between digital currents and the PLL, the
power and ground connections to the PLL are brought out separately on pins labeled
AVDD and AGND, respectively. The AVDD +2.5V supply should be derived from the
VDD INT +2.5V supply using the filter network as shown. This ensures a relatively
noise-free supply for the internal PLL. The AGND pin of the PLL should be
connected to the digital ground plane of the PC board using a short trace. The
decoupling capacitors should be routed between the AVDD pin and AGND pin using
short traces.
GROUNDING SUMMARY
There is no single grounding method which will guarantee optimum performance
100% of the time! This section has presented a number of possible options
depending upon the characteristics of the particular mixed signal devices in
question. It is helpful, however to provide for as many options as possible when
laying out the initial PC board.
It is mandatory that at least one layer of the PC board be dedicated to ground plane!
The initial board layout should provide for non-overlapping analog and digital
ground planes, but pads and vias should be provided at several locations for the
installation of back-to-back Schottky diodes or ferrite beads, if required. Pads and
vias should also be provided so that the analog and digital ground planes can be
connected together with jumpers if required.
10.30
HARDWARE DESIGN TECHNIQUES
The AGND pins of mixed-signal devices should in general always be connected to
the analog ground plane. An exception to this are DSPs which have internal phaselocked-loops (PLLs), such as the ADSP-21160 SHARC. The ground pin for the PLL
is labeled AGND, but should be connected directly to the digital ground plane for
the DSP.
GROUNDING PHILOSOPHY SUMMARY
■ There is no single grounding method which is guaranteed to work
100% of the time!
■ Different methods may or may not give the same levels of
performance.
■ At least one layer on each PC board MUST be dedicated to ground
plane!
■ Do initial layout with split analog and digital ground planes.
■ Provide pads and vias on each PC board for back-to-back
Schottky diodes and optional ferrite beads to connect the two
planes.
■ Provide "jumpers" so that DGND pins of mixed-signal devices can
be connected to AGND pins (analog ground plane) or to digital
ground plane. (AGND of PLLs in DSPs should be connected to
digital ground plane).
■ Provide pads and vias for "jumpers" so that analog and digital
ground planes can be joined together at several points on each
PC board.
■ Follow recommendations on mixed signal device data sheet.
Figure 10.24
SOME GENERAL PC BOARD LAYOUT GUIDELINES FOR MIXED-SIGNAL
SYSTEMS
It is evident that noise can be minimized by paying attention to the system layout
and preventing different signals from interfering with each other. High level analog
signals should be separated from low level analog signals, and both should be kept
away from digital signals. We have seen elsewhere that in waveform sampling and
reconstruction systems the sampling clock (which is a digital signal) is as
vulnerable to noise as any analog signal, but is as liable to cause noise as any digital
signal, and so must be kept isolated from both analog and digital systems. If clock
driver packages are used in clock distribution, only one frequency clock should be
passed through a single package. Sharing drivers between clocks of different
frequencies in the same package will produce excess jitter and crosstalk and
degrade performance.
The ground plane can act as a shield where sensitive signals cross. Figure 10.25
shows a good layout for a data acquisition board where all sensitive areas are
isolated from each other and signal paths are kept as short as possible. While real
life is rarely as tidy as this, the principle remains a valid one.
10.31
HARDWARE DESIGN TECHNIQUES
ANALOG AND DIGITAL CIRCUITS
SHOULD BE PARTITIONED ON PCB LAYOUT
SAMPLING
CLOCK GENERATOR
REFERENCE
ANALOG
ADC
CONTROL
LOGIC
BUFFER
REGISTER
DEMULTIPLEXER
DIGITAL
FILTER
DSP
OR
µP
AMPLIFIER
POWER
TIMING
CIRCUITS
MULTIPLE
ANALOG GROUNDS
INPUT
DATA
BUS
ADDRESS
BUS
BUFFER
MEMORY
MULTIPLE
GROUNDS
Figure 10.25
There are a number of important points to be considered when making signal and
power connections. First of all a connector is one of the few places in the system
where all signal conductors must run in parallel - it is therefore imperative to
separate them with ground pins (creating a faraday shield) to reduce coupling
between them.
Multiple ground pins are important for another reason: they keep down the ground
impedance at the junction between the board and the backplane. The contact
resistance of a single pin of a PCB connector is quite low (of the order of 10mΩ)
when the board is new - as the board gets older the contact resistance is likely to
rise, and the board's performance may be compromised. It is therefore well
worthwhile to allocate extra PCB connector pins so that there are many ground
connections (perhaps 30-40% of all the pins on the PCB connector should be ground
pins). For similar reasons there should be several pins for each power connection,
although there is no need to have as many as there are ground pins.
Manufacturers of high performance mixed-signal ICs like Analog Devices offer
evaluation boards to assist customers in their initial evaluations and layout. ADC
evaluation boards generally contain an on-board low-jitter sampling clock oscillator,
output registers, and appropriate power and signal connectors. They also may have
additional support circuitry such as the ADC input buffer amplifier and external
reference.
10.32
HARDWARE DESIGN TECHNIQUES
The layout of the evaluation board is optimized in terms of grounding, decoupling,
and signal routing and can be used as a model when laying out the ADC PC board
in the system. The actual evaluation board layout is usually available from the ADC
manufacturer in the form of computer CAD files (Gerber files). In many cases, the
layout of the various layers appears on the data sheet for the device.
10.33
HARDWARE DESIGN TECHNIQUES
REFERENCES ON GROUNDING:
1.
William C. Rempfer, Get All the Fast ADC Bits You Pay For,
Electronic Design, Special Analog Issue, June 24, 1996, p.44.
2.
Mark Sauerwald, Keeping Analog Signals Pure in a Hostile Digital
World, Electronic Design, Special Analog Issue, June 24, 1996, p.57.
3.
Jerald Grame and Bonnie Baker, Design Equations Help Optimize
Supply Bypassing for Op Amps, Electronic Design, Special Analog
Issue, June 24, 1996, p.9.
4.
Jerald Grame and Bonnie Baker, Fast Op Amps Demand More Than
a Single-Capacitor Bypass, Electronic Design, Special Analog Issue,
November 18, 1996, p.9.
5.
Walt Kester and James Bryant, Grounding in High Speed Systems,
High Speed Design Techniques, Analog Devices, 1996, Chapter 7, p. 7-27.
6.
Jeffrey S. Pattavina, Bypassing PC Boards: Thumb Your Nose at Rules
of Thumb, EDN, Oct. 22, 1998, p.149.
7.
Henry Ott, Noise Reduction Techniques in Electronic Systems,
Second Edition, New York, John Wiley and Sons, 1988.
8.
Howard W. Johnson and Martin Graham, High-Speed Digital Design,
PTR Prentice Hall, 1993.
9.
Paul Brokaw, An I.C. Amplifier User's Guide to Decoupling, Grounding
and Making Things Go Right for a Change, Application Note,
Analog Devices, Inc., http://www.analog.com.
10.
Walt Kester, A Grounding Philosophy for Mixed-Signal Systems,
Electronic Design Analog Applications Issue, June 23, 1997, p. 29.
11.
Ralph Morrison, Grounding and Shielding Techniques, Fourth Edition,
John Wiley, 1998.
12.
Ralph Morrison, Solving Interference Problems in Electronics,
John Wiley, 1995.
13.
C. D. Motchenbacher and J. A. Connelly, Low Noise Electronic System
Design, John Wiley, 1993.
14.
Crystal Oscillators: MF Electronics, 10 Commerce Drive, New Rochelle,
NY, 10801, 914-576-6570.
15.
Mark Montrose, EMC and the Printed Circuit Board, IEEE Press,
1999 (IEEE Order Number PC5756).
10.34
HARDWARE DESIGN TECHNIQUES
DIGITAL ISOLATION TECHNIQUES
Walt Kester
One way to break ground loops is to use isolation techniques. Analog isolation
amplifiers find many applications where a high degree of isolation is required, such
as in medical instrumentation. Digital isolation techniques offer a reliable method
of transmitting digital signals over interfaces without introducing ground noise.
Optocouplers (also called optoisolators) are useful and available in a wide variety of
styles and packages. A typical optocoupler based on an LED and a phototransitor is
shown in Figure 10.26. A current of approximately 10mA is applied to an LED
transmitter, and the light output is received by a phototransistor. The light
produced by the LED is sufficient to saturate the phototransistor. Isolation of 5000V
RMS to 7000V RMS is common. Although excellent for digital signals, optocouplers
are too nonlinear for most analog applications. One should also realize that since
the phototransistor is operated in a saturated mode, rise and fall-times can range
from 10µs to 20µs in slower devices, thereby limiting applications at high speeds.
DIGITAL ISOLATION USING
LED / PHOTOTRANSISTOR OPTOCOUPLERS
VDD1 (5V) 425Ω
VIN
CMOS
GATE
GND1
10kΩ
IOUT
IIN
HIGH VOLTAGE
ISOLATION BARRIER
VDD2 (5V)
VOUT
GND2
■ Uses Light for Transmission Over a High Voltage Barrier
■ The LED is the Transmitter, and the Phototransistor is the Receiver
■ High Voltage Isolation: 5000V to 7000V RMS
■ Non-Linear -- Best for Digital or Frequency Information
■ Rise and Fall-times can be 10 to 20µs in Slower Devices
■ Example: Siemens ILQ-1 Quad (http://www.siemens.com)
Figure 10.26
A faster optocoupler architecture is shown in Figure 10.27 and is based on an LED
and a photodiode. The LED is again driven with a current of approximately 10mA.
This produces a light output sufficient generate enough current in the receiving
photodiode to develop a valid high logic level at the output of the transimpedance
amplifier. Speed can vary widely between optocouplers, and the fastest ones have
propagation delays of 20ns typical, and 40ns maximum, and can handle data rates
10.35
HARDWARE DESIGN TECHNIQUES
up to 25MBd for NRZ data. This corresponds to a maximum square wave operating
frequency of 12.5MHz, and a minimum allowable passable pulse width of 40ns.
DIGITAL ISOLATION USING
LED / PHOTODIODE OPTOCOUPLERS
VDD1
VDD2
VIN
VOUT
(Data In)
(Data Out)
GND1
GND2
■
■
■
■
■
■
■
+5V Supply Voltage
2500V RMS I/O Withstand Voltage
Logic Signal Frequency: 12.5MHz Maximum
25MBd Maximum Data Rate
40ns Maximum Propagation Delay
9ns Typical Rise/Fall Time
Example: Agilent HCPL-7720
(http://www.semiconductor.agilent.com)
Figure 10.27
The ADuM1100A and ADuM1100B are digital isolators based on Analog Devices’
µmIsolation™ (micromachined isolation) technology. Combining high speed CMOS
and monolithic air core transformer technology, these isolation components provide
outstanding performance characteristics superior to the traditional optocouplers
previously described.
Configured as pin-compatible replacements for existing high speed optocouplers, the
ADuM1100A and ADuM1100B support data rates as high as 25MBd and 100MBb,
respectively. A functional diagram of the devices is shown in Figure 10.28.
Both the ADuM1100A and ADuM1100B operate at either 3.3V or 5V supply
voltages, have propagation delays < 10ns, edge asymmetry of <2ns, and rise and
fall-times < 2ns. They operate at very low power, less than 600µA of quiescent
current (sum of both sides) and a dynamic current of less than 230µA per MBd of
data rate. Unlike common transformer implementations, the parts provide DC
correctness with a patented refresh feature which continuously updates the output
signal.
10.36
HARDWARE DESIGN TECHNIQUES
ADuM1100A/ADuM1100B DIGITAL ISOLATORS
VDD1
VDD2
VIN
ENCODE
DECODE
VOUT
(Data Out)
(Data In)
UPDATE
WATCH
GND1
GND2
■
■
■
■
■
■
■
+5V/+3.3V Supply Voltage
2500V RMS I/O Withstand Voltage
25MBd Maximum Data Rate (ADuM1100A)
100 MBd Maximum Data Rate (ADuM1100B)
10ns Maximum Propagation Delay
2ns Typical Rise/Fall Time
Pin Compatible with Popular Optocouplers
Figure 10.28
The AD260/AD261 family of digital isolators isolates five digital control signals
to/from high speed DSPs, microcontrollers, or microprocessors. The AD260 also has
a 1.5W transformer for a 3.5kV RMS isolated external DC/DC power supply circuit.
Each line of the AD260 can handle digital signals up to 20MHz (40MBd) with a
propagation delay of only 14ns which allows for extremely fast data transmission.
Output waveform symmetry is maintained to within ±1ns of the input so the AD260
can be used to accurately isolate time-based pulse width modulator (PWM) signals.
A simplified schematic of one channel of the AD260/AD261 is shown in Figure
10.29. The data input is passed through a schmitt trigger circuit, through a latch,
and a special transmitter circuit which differentiates the edges of the digital input
signal and drives the primary winding of a proprietary transformer with a "sethigh/set-low" signal. The secondary of the isolation transformer drives a receiver
with the same "set-hi/set-low" data which regenerates the original logic waveform.
An internal circuit operates in the background which interrogates all inputs about
every 5µs and in the absence of logic transitions, sends appropriate "set-hi/set-low"
data across the interface. Recovery time from a fault condition or at power-up is
thus between 5µs and 10µs.
The power transformer (available on the AD260) is designed to operate between
150kHz and 250kHz and will easily deliver more than 1W of isolated power when
driven push-pull (5V) on the transmitter side. Different transformer taps, rectifier
10.37
HARDWARE DESIGN TECHNIQUES
and regulator schemes will provide combinations of ±5V, 15V, 24V, or even 30V or
higher. The output voltage when driven with a low voltage-drop drive will be
37V p-p across the entire secondary with a 5V push-pull drive.
AD260/AD261 DIGITAL ISOLATORS
DATA
IN
SCHMITT
TRIGGER
LATCH
D
TRI STATE
XMTR
RCVR
DATA
OUT
E
ENABLE
ENABLE
CONTINUOUS
UPDATE
CIRCUIT
ISOLATED POWER
XFMR (AD260)
37V p-p, 1.5W
NOTE: SINGLE DATA CHANNEL SHOWN
3500V RMS ISOLATION BARRIER
(AD260B/AD261B)
Figure 10.29
AD260/AD261 DIGITAL ISOLATOR KEY SPECIFICATIONS
■ Isolation Test Voltage to 3500V RMS (AD260B/AD261B)
■ Five Isolated Digital Lines Available in 6 Input/Output Configurations
■ Logic Signal Frequency: 20MHz Max.
■ Data Rate: 40MBd Max.
■ Isolated Power Transformer: 37V p-p, 1.5W (AD260)
■ Waveform Edge Transmission Symmetry: ±1ns
■ Propagation Delay: 14ns
■ Rise and Fall-Times < 5ns
Figure 10.30
10.38
HARDWARE DESIGN TECHNIQUES
POWER SUPPLY NOISE REDUCTION AND
FILTERING
Walt Jung, Walt Kester, Bill Chestnut
Precision analog circuitry has traditionally been powered from well regulated, low
noise linear power supplies. During the last decade however, switching power
supplies have become much more common in electronic systems. As a consequence,
they also are being used for analog supplies. Good reasons for the general popularity
include their high efficiency, low temperature rise, small size, and light weight.
In spite of these benefits, switchers do have drawbacks, most notably high output
noise. This noise generally extends over a broad band of frequencies, resulting in
both conducted and radiated noise, as well as unwanted electric and magnetic
fields. Voltage output noise of switching supplies are short-duration voltage
transients, or spikes. Although the fundamental switching frequency can range
from 20kHz to 1MHz, the spikes can contain frequency components extending to
100MHz or more. While specifying switching supplies in terms of RMS noise is
common vendor practice, as a user you should also specify the peak (or p-p)
amplitudes of the switching spikes, at the output loading of your system.
The following section discusses filter techniques for rendering a switching regulator
output analog ready, that is sufficiently quiet to power precision analog circuitry
with relatively small loss of DC terminal voltage. The filter solutions presented are
generally applicable to all power supply types incorporating switching element(s) in
their energy path. This includes various DC-DC converters as well as popular 5V
(PC type) supplies.
An understanding of the EMI process is necessary to understand the effects of
supply noise on analog circuits and systems. Every interference problem has a
source, a path, and a receptor [Reference 1]. In general, there are three methods for
dealing with interference. First, source emissions can be minimized by proper
layout, pulse-edge rise time control/reduction, filtering, and proper grounding.
Second, radiation and conduction paths should be reduced through shielding and
physical separation. Third, receptor immunity to interference can be improved, via
supply and signal line filtering, impedance level control, impedance balancing, and
utilizing differential techniques to reject undesired common-mode signals. This
section focuses on reducing switching power supply noise with external post filters.
Tools useful for combating high frequency switcher noise are shown by Figure 10.31.
They differ in electrical characteristics as well as their practicality towards noise
reduction, and are listed roughly in an order of priorities. Of these tools, L and C
are the most powerful filter elements, and are the most cost-effective, as well as
small in size.
10.39
HARDWARE DESIGN TECHNIQUES
SWITCHING REGULATOR NOISE REDUCTION TOOLS
■ Capacitors
■ Inductors
■ Ferrites
■ Resistors
■ Linear Post Regulation
■ Proper Layout and Grounding Techniques
■ PHYSICAL SEPARATION FROM SENSITIVE
ANALOG CIRCUITS!!
Figure 10.31
Capacitors are probably the single most important filter component for switchers.
There are many different types of capacitors, and an understanding of their
individual characteristics is absolutely mandatory in the design of effective practical
supply filters. There are generally three classes of capacitors useful in 10kHz100MHz filters, broadly distinguished as the generic dielectric types; electrolytic,
organic, film, and ceramic. These can in turn can be further sub-divided. A
thumbnail sketch of capacitor characteristics is shown in the chart of Figure 10.32.
TYPES OF CAPACITORS
Aluminum
Electrolytic
(General
Purpose)
Aluminum
Electrolytic
(Switching
Type)
Tantalum
Electrolytic
OS-CON
Electrolytic
Polyester
(Stacked
Film)
Ceramic
(Multilayer)
Size
100 µF
120 µF
120 µF
100 µF
1 µF
0.1 µF
Rated
Voltage
25 V
25 V
20 V
20 V
400 V
50 V
0.6 Ω @
100 kHz
0.18 Ω @
100 kHz
0.12 Ω @
100 kHz
0.02 Ω @
100 kHz
0.11 Ω @
1 MHz
0.12 Ω @
1 MHz
≅ 100 kHz
≅ 500 kHz
≅ 1 MHz
≅ 1 MHz
≅ 10 MHz
≅ 1 GHz
ESR
Operating
Frequency
(*)
(*) Upper frequency strongly size and package dependent
Figure 10.32
10.40
HARDWARE DESIGN TECHNIQUES
With any dielectric, a major potential filter loss element is ESR (equivalent series
resistance), the net parasitic resistance of the capacitor. ESR provides an ultimate
limit to filter performance, and requires more than casual consideration, because it
can vary both with frequency and temperature in some types. Another capacitor loss
element is ESL (equivalent series inductance). ESL determines the frequency where
the net impedance characteristic switches from capacitive to inductive. This varies
from as low as 10kHz in some electrolytics to as high as 100MHz or more in chip
ceramic types. Both ESR and ESL are minimized when a leadless package is used.
All capacitor types mentioned are available in surface mount packages, preferable
for high speed uses.
The electrolytic family provides an excellent, cost-effective low-frequency filter
component, because of the wide range of values, a high capacitance-to-volume ratio,
and a broad range of working voltages. It includes general purpose aluminum
electrolytic types, available in working voltages from below 10V up to about 500V,
and in size from 1 to several thousand µF (with proportional case sizes). All
electrolytic capacitors are polarized, and thus cannot withstand more than a volt or
so of reverse bias without damage. They also have relatively high leakage currents
(up to tens of µA, and strongly dependent upon design specifics).
A subset of the general electrolytic family includes tantalum types, generally
limited to voltages of 100V or less, with capacitance of up to 500µF [Reference 3]. In
a given size, tantalums exhibit a higher capacitance-to-volume ratios than do
general purpose electrolytics, and have both a higher frequency range and lower
ESR. They are generally more expensive than standard electrolytics, and must be
carefully applied with respect to surge and ripple currents.
A subset of aluminum electrolytic capacitors is the switching type, designed for
handling high pulse currents at frequencies up to several hundred kHz with low
losses [Reference 4]. This capacitor type competes directly with tantalums in high
frequency filtering applications, with the advantage of a broader range of values.
A more specialized high performance aluminum electrolytic capacitor type uses an
organic semiconductor electrolyte [Reference 5]. The OS-CON capacitors feature
appreciably lower ESR and higher frequency range than do other electrolytic types,
with an additional feature of low low-temperature ESR degradation.
Film capacitors are available in a very broad range of values and an array of
dielectrics, including polyester, polycarbonate, polypropylene, and polystyrene.
Because of the low dielectric constant of these films, their volumetric efficiency is
quite low, and a 10µF/50V polyester capacitor (for example) is actually the size of
your hand. Metalized (as opposed to foil) electrodes do help to reduce size, but even
the highest dielectric constant units among film types (polyester, polycarbonate) are
still larger than any electrolytic, even using the thinnest films with the lowest
10.41
HARDWARE DESIGN TECHNIQUES
voltage ratings (50V). Where film types excel is in their low dielectric losses, a factor
which may not necessarily be a practical advantage for filtering switchers. For
example, ESR in film capacitors can be as low as 10mΩ or less, and the behavior of
films generally is very high in terms of Q. In fact, this can cause problems of
spurious resonance in filters, requiring damping components.
Film capacitors using a wound layer-type construction can be inductive. This can
limit their effectiveness for high frequency filtering. Obviously, only non-inductively
made film caps are useful for switching regulator filters. One specific style which is
non-inductive is the stacked-film type, where the capacitor plates are cut as small
overlapping linear sheet sections from a much larger wound drum of dielectric/plate
material. This technique offers the low inductance attractiveness of a plate sheet
style capacitor with conventional leads [see References 4, 5, 6]. Obviously, minimal
lead length should be used for best high frequency effectiveness. Very high current
polycarbonate film types are also available, specifically designed for switching
power supplies, with a variety of low inductance terminations to minimize ESL
[Reference 7].
Dependent upon their electrical and physical size, film capacitors can be useful at
frequencies to well above 10MHz. At the highest frequencies, only stacked film
types should be considered. Some manufacturers are now supplying film types in
leadless surface mount packages, which eliminates the lead length inductance.
Ceramic is often the capacitor material of choice above a few MHz, due to its
compact size, low loss, and availability up to several µF in the high-K dielectric
formulations (X7R and Z5U), at voltage ratings up to 200V [see ceramic families of
Reference 3]. NP0 (also called COG) types use a lower dielectric constant
formulation, and have nominally zero TC, plus a low voltage coefficient (unlike the
less stable high-K types). NP0 types are limited to values of 0.1µF or less, with
0.01µF representing a more practical upper limit.
Multilayer ceramic “chip caps” are very popular for bypassing/ filtering at 10MHz or
higher, simply because their very low inductance design allows near optimum RF
bypassing. For smaller values, ceramic chip caps have an operating frequency range
to 1GHz. For high frequency applications, a useful selection can be ensured by
selecting a value which has a self-resonant frequency above the highest frequency of
interest.
All capacitors have some finite ESR. In some cases, the ESR may actually be helpful
in reducing resonance peaks in filters, by supplying “free” damping. For example, in
most electrolytic types, a nominally flat broad series resonance region can be noted
by the impedance vs. frequency plot. This occurs where |Z| falls to a minimum
level, nominally equal to the capacitor’s ESR at that frequency. This low Q
resonance can generally cover a relatively wide frequency range of several octaves.
10.42
HARDWARE DESIGN TECHNIQUES
Contrasted to the very high Q sharp resonances of film and ceramic caps, the low Q
behavior of electrolytics can be useful in controlling resonant peaks.
In most electrolytic capacitors, ESR degrades noticeably at low temperature, by as
much as a factor of 4-6 times at –55°C vs. the room temperature value. For circuits
where ESR is critical to performance, this can lead to problems. Some specific
electrolytic types do address this problem, for example within the HFQ switching
types, the –10°C ESR at 100kHz is no more than 2× that at room temperature. The
OSCON electrolytics have a ESR vs. temperature characteristic which is relatively
flat.
As noted, all real capacitors have parasitic elements which limit their performance.
The equivalent electrical network representing a real capacitor models both ESR
and ESL as well as the basic capacitance, plus some shunt resistance (see Figure
10.33). In such a practical capacitor, at low frequencies the net impedance is almost
purely capacitive. At intermediate frequencies, the net impedance is determined by
ESR, for example about 0.12Ω to 0.4Ω at 125kHz, for several types. Above about
1MHz these capacitor types become inductive, with impedance dominated by the
effect of ESL. All electrolytics will display impedance curves similar in general
shape to that of Figure 10.34. The minimum impedance will vary with the ESR, and
the inductive region will vary with ESL (which in turn is strongly affected by
package style).
CAPACITOR EQUIVALENT CIRCUIT
AND PULSE RESPONSE
i
IPEAK = 1A
INPUT
CURRENT
v
Ω
ESR = 0.2Ω
di
1A
=
dt
100ns
Equivalent f = 3.5MHz
0
ESL = 20nH
VPEAK = ESL •
C = 100µF
OUTPUT
VOLTAGE
Ω
XC = 0.0005Ω
@ 3.5MHz
di
+ ESR • IPEAK
dt
= 400mV
ESR • IPEAK = 200mV
0
Figure 10.33
10.43
HARDWARE DESIGN TECHNIQUES
ELECTROLYTIC CAPACITOR
IMPEDANCE VERSUS FREQUENCY
C (100µF)
REGION
LOG
|Z|
ESL (20nH)
REGION
ESR (0.2Ω)
Ω)
REGION
Ω
ESR = 0.2Ω
10kHz
1MHz
LOG FREQUENCY
Figure 10.34
Regarding inductors, Ferrites (non-conductive ceramics manufactured from the
oxides of nickel, zinc, manganese, or other compounds) are extremely useful in
power supply filters [Reference 9]. At low frequencies (<100kHz), ferrites are
inductive; thus they are useful in low-pass LC filters. Above 100kHz, ferrites
become resistive, an important characteristic in high-frequency filter designs.
Ferrite impedance is a function of material, operating frequency range, DC bias
current, number of turns, size, shape, and temperature. Figure 10.35 summarizes a
number of ferrite characteristics, and Figure 10.36 shows the impedance
characteristic of several ferrite beads from Fair-Rite (http://www.fair-rite.com).
Several ferrite manufacturers offer a wide selection of ferrite materials from which
to choose, as well as a variety of packaging styles (see References 10 and 11). A
simple form is the bead of ferrite material, a cylinder of the ferrite which is simply
slipped over the power supply lead to the decoupled stage. Alternately, the leaded
ferrite bead is the same bead, pre-mounted on a length of wire and used as a
component (see Reference 11). More complex beads offer multiple holes through the
cylinder for increased decoupling, plus other variations. Surface mount beads are
also available.
10.44
HARDWARE DESIGN TECHNIQUES
FERRITES SUITABLE FOR HIGH FREQUENCY FILTERS
■ Ferrites Good for Frequencies Above 25kHz
■ Many Sizes and Shapes Available Including Leaded "Resistor
Style"
■ Ferrite Impedance at High Frequencies Primarily Resistive -Ideal for HF Filtering
■ Low DC Loss: Resistance of Wire Passing Through Ferrite is
Very Low
■ High Saturation Current Versions Available
■ Choice Depends Upon:
◆ Source and Frequency of Interference
◆ Impedance Required at Interference Frequency
◆ Environmental: Temperature, AC and DC Field Strength,
Size / Space Available
■
Always Test the Design!
Figure 10.35
IMPEDANCE OF FERRITE BEADS
80
#43
MATERIAL
#64
MATERIAL
60
Z
Ω
#73
MATERIAL
40
20
0
1
10
100
1000
FREQUENCY (MHz)
Courtesy: Fair-Rite Products Corp, Wallkill, NY
(http://www.fair-rite.com)
Figure 10.36
10.45
HARDWARE DESIGN TECHNIQUES
PSpice ferrite models for Fair-Rite materials are available, and allow ferrite
impedance to be estimated [see Reference 12]. These models have been designed to
match measured impedances rather than theoretical impedances.
A ferrite’s impedance is dependent upon a number of inter-dependent variables, and
is difficult to quantify analytically, thus selecting the proper ferrite is not
straightforward. However, knowing the following system characteristics will make
selection easier. First, determine the frequency range of the noise to be filtered.
Second, the expected temperature range of the filter should be known, as ferrite
impedance varies with temperature. Third, the peak DC current flowing through
the ferrite must be known, to ensure that the ferrite does not saturate. Although
models and other analytical tools may prove useful, the general guidelines given
above, coupled with some experimentation with the actual filter connected to the
supply output under system load conditions, should lead to a proper ferrite
selection.
Using proper component selection, low and high frequency band filters can be
designed to smooth a noisy switcher’s DC output to produce an analog ready 5V
supply. It is most practical to do this over two (and sometimes more) stages, each
stage optimized for a range of frequencies. A basic stage can be used to carry all of
the DC load current, and filter noise by 60dB or more up to a 1-10MHz range. This
larger filter is used as a card entry filter providing broadband filtering for all power
entering a PC card. Smaller, more simple local filter stages are also used to provide
higher frequency decoupling right at the power pins of individual stages.
SWITCHING REGULATOR EXPERIMENTS
In order to better understand the challenge of filtering switching regulators, a series
of experiments were conducted with a representative device, the ADP1148
synchronous buck regulator with a 9V input and a 3.3V/1A output.
In addition to observing typical input and output waveforms, the objective of these
experiments was to reduce the output ripple to less than 10mV peak-to-peak, a
value suitable for driving most analog circuits.
Measurements were made using a Tektronix wideband digitizing oscilloscope with
the input bandwidth limited to 20MHz so that the ripple generated by the switching
regulators could be more readily observed. In a system, power supply ripple
frequencies above 20MHz are best filtered locally at each IC power pin with a low
inductance ceramic capacitor and perhaps a series-connected ferrite bead.
Probing techniques are critical for accurate ripple measurements. A standard
passive 10X probe was used with a "bayonet" probe tip adapter for making the
ground connection as short as possible (see Figure 10.37). Use of the "ground clip
lead" is not recommended in making this type of measurement because the lead
length in the ground connection forms an unwanted inductive loop which picks up
high frequency switching noise, thereby corrupting the signal being measured.
10.46
HARDWARE DESIGN TECHNIQUES
PROPER PROBING TECHNIQUES
PROBE
"GROUND CLIP"
CONNECTOR
SLIP-ON
"BAYONET"
GROUND
ADAPTER
"GROUND CLIP"
LEAD
(DO NOT USE!!)
SIGNAL
CONTACT
GROUND PLANE
CONTACT
IC
Figure 10.37
Note: Schematic representation of proper physical grounding is almost impossible. In
all the following circuit schematics, the connections to ground are made to the
ground plane using the shortest possible connecting path, regardless of how they are
indicated in the actual circuit schematic diagram.
The circuit for the ADP1148 9V to 3.3V/1A buck regulator is shown in Figure 10.38.
The output waveform of the ADP1148 buck regulator is shown in Figure 10.39. The
fundamental switching frequency is approximately 150kHz, and the output ripple is
approximately 40mV.
Adding an output filter consisting of a 50µH inductor and a 100µF leaded tantalum
capacitor reduced the ripple to approximately 3mV as shown in Figure 10.40.
Linear regulators are often used following switching regulators for better regulation
and lower noise. Low dropout (LDO) regulators such as the ADP3310 are desirable
in these applications because they require only a small input-to-output series
voltage to maintain regulation. This minimizes power dissipation in the pass device
and may eliminate the need for a heat sink. Figure 10.41 shows the ADP1148 buck
regulator configured for a 9V input and a 3.75V/1A output. The output drives an
ADP3310 linear LDO regulator configured for 3.75V input and 3.3V/1A output. The
input and output of the ADP3310 is shown in Figure 10.42. Notice that the
regulator reduces the ripple from 40mV to approximately 5mV.
10.47
HARDWARE DESIGN TECHNIQUES
ADP1148 BUCK REGULATOR CIRCUIT
+
1µF
VIN, 9V
VIN
10nF
220µF/25V
IRF7204
C1
INT VCC
P-DRIVE
P-CH
ADP1148-3.3
L, 50µH
RSENSE
0.1Ω
Ω
SHUTDOWN
ITH
SENSE (+)
CT
SENSE (–)
RC, 1kΩ
Ω
CC
CT
3300pF
470pF
C2
+
1000pF
N-CH
N-DRIVE
SGND
VOUT
3.3V/1A
10BQ040
100µF
20V
PGND
IRF7403
C1 = 220µF/25V GEN PURPOSE AL ELECTROLYTIC
+ 1µF CERAMIC
L=COILTRONICS CTX-50-4
C2 = 100µF/20V LEADED TANTALUM, KEMET T356-SERIES, ESR = 0.6Ω
Ω
Figure 10.38
ADP1148 BUCK OUTPUT WAVEFORM
VOUT
3.3V
1A
VIN
9V
+
1µF 220µF
25V
C1 = 220 µF
+1 µF
ADP1148
BUCK
REG
CIRCUIT
40mV p-p
+
100µF
20V
C2 = 100µF/20V
VERTICAL SCALE:
HORIZ. SCALE:
10mV / DIV
5µs / DIV
C1 = 1µF CERAMIC + 220µF/25V GENERAL PURPOSE AL ELECTROLYTIC
C2 = 100µF/20V LEADED TANTALUM, KEMET T356-SERIES (ESR = 0.6Ω
Ω)
Figure 10.39
10.48
HARDWARE DESIGN TECHNIQUES
ADP1148 BUCK FILTERED OUTPUT
LF
50µH
VIN
9V
ADP1148
BUCK
REG
CIRCUIT
+
1µF 220µF
25V
C1 = 220 µF
+1 µF
VOUT
3.3V
1A
3mV p-p
+
+
100µF
20V
CF
100µF
20V
C2 = 100µF/20V
VERTICAL SCALE:
HORIZ. SCALE:
10mV / DIV
5µs / DIV
C1 = 1µF CERAMIC + 220µF/25V GENERAL PURPOSE AL ELECTROLYTIC
C2 = 100µF/20V LEADED TANTALUM, KEMET T356-SERIES (ESR = 0.6Ω
Ω)
OUTPUT FILTER
LF=COILTRONICS CTX-50-4
CF = 100µF/20V LEADED TANTALUM, KEMET T356-SERIES
Figure 10.40
ADP1148 BUCK REGULATOR DRIVING
ADP3310 LOW DROPOUT REGULATOR
VIN, 9V
WAVEFORMS
1µF
VIN
10nF
IRF7204
P-DRIVE
IFR7404
C1
INT VCC
P-CH
ADP1148
+
220µF
35V
L, 68µH
RSENSE
0.1Ω
Ω
3.75V
SD
ITH
IN
SENSE (+)
RC, 1kΩ
Ω
CC
2200pF
CT
470pF
C2
SENSE (–)
N-CH
N-DRIVE
SGND
OUT
ADP3310-3.3
1000pF
CT
GATE
10BQ040
FB PGND
+
100µF
20V
GND
3.3V
1A
C3
10µF
35V
R1
20kΩ
Ω
R2
10kΩ
Ω
IRF7403
Figure 10.41
10.49
HARDWARE DESIGN TECHNIQUES
WAVEFORMS FOR ADP1148 BUCK REGULATOR
DRIVING ADP3310 LOW DROPOUT REGULATOR
ADP1148 OUTPUT
(ADP3310 INPUT)
ADP3310 OUTPUT
40mV p-p
VERTICAL SCALE: 10mV/DIV
HORIZ. SCALE:
5µs/DIV
5mV p-p
VERTICAL SCALE: 10mV/DIV
HORIZ. SCALE:
5µs/DIV
Figure 10.42
There are many tradeoffs in designing power supply filters. The success of any
filter circuit is highly dependent upon a compact layout and the use of a large area
ground plane. As has been stated earlier, all connections to the ground plane should
be made as short as possible to minimize parasitic resistance and inductance.
Output ripple can be reduced by the addition of low ESL/ESR capacitors to the
output. However, it may be more efficient to use an LC filter to accomplish the
ripple reduction. In any case, proper component selection is critical. The inductor
should not saturate under the maximum load current, and its DC resistance should
be low enough as not to induce significant voltage drop. The capacitors should have
low ESL and ESR and be rated to handle the required ripple current.
Low dropout linear post regulators provide both ripple reduction as well as better
regulation and can be effective, provided the sacrifice in efficiency is not excessive.
Finally, it is difficult to predict the output ripple current analytically, and there is
no substitute for a prototype using the real-world components. Once the filter is
proven to provide the desired ripple attenuation (with some added safety margin),
care must be taken that parts substitutions or vendor changes are not made in the
final production units without first testing them in the circuit for equivalent
performance.
10.50
HARDWARE DESIGN TECHNIQUES
SWITCHING SUPPLY FILTER SUMMARY
■ Proper Layout and Grounding (using Ground Plane) Mandatory
■ Low ESL/ESR Capacitors Give Best Results
■ Parallel Capacitors Lower ESR/ESL and Increase Capacitance
■ External LC Filters Very Effective in Reducing Ripple
■ Linear Post Regulation Effective for Noise Reduction and Best
Regulation
■ Completely Analytical Approach Difficult, Prototyping is
Required for Optimum Results
■ Once Design is Finalized, Do Not Switch Vendors or Use Parts
Substitutions Without First Verifying Their Performance in
Circuit
■ High Frequency Localized Decoupling at IC Power Pins is Still
Required
Figure 10.43
LOCALIZED HIGH FREQUENCY POWER SUPPLY FILTERING
The LC filters described in the previous section are useful in filtering switching
regulator outputs. However, it may be desirable to place similar filters on the
individual PC boards where the power first enters the board. Of course, if the
switching regulator is placed on the PC board, then the LC filter should be an
integral part of the regulator design.
Localized high frequency filters may also be required at each IC power pin (see
Figure 10.44). Surface mount ceramic capacitors are ideal choices because of their
low ESL. It is important to make the connections to the power pin and the ground
plane as short as possible. In the case of the ground connection, a via directly to the
ground plane is the shortest path. Routing the capacitor ground connection to
another ground pin on the IC is not recommended due to the added inductance of
the trace. In some cases, a ferrite bead in series with the power connection may also
be desirable.
10.51
HARDWARE DESIGN TECHNIQUES
LOCALIZED DECOUPLING TO GROUND PLANE
USING SHORTEST PATH
CORRECT
POWER
SUPPLY
TRACE
OPTIONAL
FERRITE BEADS
DECOUPLING
CAPACITOR
INCORRECT
POWER
SUPPLY
TRACE
DECOUPLING
CAPACITOR
V+
IC
V+
VIAS TO
GROUND
PLANE
GND
IC
PCB
TRACE
GND
VIA TO
GROUND
PLANE
Figure 10.44
The following list summarizes the switching power supply filter layout/construction
guidelines which will help ensure that the filter does the best possible job:
(1) Pick the highest electrical value and voltage rating for filter capacitors which is
consistent with budget and space limits. This minimizes ESR, and maximizes filter
performance. Pick chokes for low ∆L at the rated DC current, as well as low DCR.
(2) Use short and wide PCB tracks to decrease voltage drops and minimize
inductance. Make track widths at least 200 mils for every inch of track length for
lowest DCR, and use 1 oz or 2 oz copper PCB traces to further reduce IR drops and
inductance.
(3) Use short leads or better yet, leadless components, to minimize lead inductance.
This minimizes the tendency to add excessive ESL and/or ESR. Surface mount
packages are preferred. Make all connections to the ground plane as short as
possible.
(4) Use a large-area ground plane for minimum impedance.
(5) Know what your components do over frequency, current and temperature
variations! Make use of vendor component models for the simulation of prototype
designs, and make sure that lab measurements correspond reasonably with the
10.52
HARDWARE DESIGN TECHNIQUES
simulation. While simulation is not absolutely necessary, it does instill confidence in
a design when correlation is achieved (see Reference 15).
HIGH DENSITY DSP LOCALIZED DECOUPLING CONSIDERATIONS
High pin-count DSP packages require special consideration with respect to localized
decoupling due to their high digital transient currents. Typical decoupling
arrangements are shown in Figure 10.45. The surface mount capacitors are placed
on the top side of the PC board in Figure 10.45A. For the SHARC-family, eight
0.02µF ceramic capacitors are recommended. They should be placed as close to the
package as possible. The connections to the VDD pins should be as short as possible
using wide traces. The connections to ground should be made directly to the ground
plane with vias. A less desirable method is shown in Figure 10.45B, where the
capacitors are mounted on the back side of the PC board underneath the footprint of
the package. If the ground plane underneath the package footprint is perforated
with many signal vias, the capacitor return transient current must flow to the
outside ground plane, which may be poorly connected to the inside ground plane due
to the vias.
DECOUPLING HIGH PIN-COUNT DSPs
IN PQFP PACKAGES
A
B
TOP VIEW
OF PACKAGE
TOP VIEW
OF PACKAGE
CAPACITORS ON BACK SIDE OF PC BOARD
CAPACITORS ON TOP SIDE OF PC BOARD
Figure 10.45
10.53
HARDWARE DESIGN TECHNIQUES
The PC board for a ball grid array (BGA) package is shown in Figure 10.46. Note
that all connections to the balls must be made using vias to other layers of the
board. The “dogbone” pattern shown is often used for the BGA packages. The
shaded area indicates the location of the solder mask. As in the case of PQFP
packages, the localized decoupling capacitors should be placed as close as possible to
the package with short connections to the VDD pins and direct connections to vias to
the ground plane layer.
DECOUPLING HIGH PIN-COUNT DSPs
IN BALL GRID ARRAY (BGA) PACKAGES
TOP VIEW
OF PC BOARD
WITHOUT
PACKAGE
"DOGBONE" PAD
VIA
SOLDER MASK
CROSS SECTION
VIEW
BGA PACKAGE
SOLDER BALLS
MULTILAYER
PC BOARD
VIAS
Figure 10.46
The ADSP-21160 400-ball BGA 27mm by 27mm package approximate power and
ground assignments are shown in Figure 10.47. The ball pitch is 1.27mm.
Approximately 84 balls are used in the center of the pattern for ground connections.
The connections to the core voltage (40 balls) and the external voltage (46 balls)
surround the ground balls. The remaining outer balls are used for the various
signals.
The centrally located ground balls serve a dual function. Their primary function is
make a low impedance connection directly to the ground plane layer. Their
secondary function is to conduct the package heat to the ground plane layer which
also acts as a heat sink, since the device must dissipate about 2.5W under average
operating conditions. The addition of an external heatsink as shown lowers the
junction-to-ambient thermal resistance even further.
10.54
HARDWARE DESIGN TECHNIQUES
ADSP-21160 DSP 400-PIN
PBGA PACKAGE BALL LOCATIONS
BALL LOCATIONS
ARE APPROXIMATE,
ALL BALLS NOT SHOWN
GROUND BALLS
(84)
VOLTAGE BALLS
(86)
PACKAGE SIZE:
27mm X 27mm
BALL PITCH: 1.27mm
SIGNAL BALLS
MULTILAYER
PC BOARD
GROUND AND HEATSINK
Figure 10.47
10.55
HARDWARE DESIGN TECHNIQUES
REFERENCES: NOISE REDUCTION AND FILTERING
1.
EMC Design Workshop Notes, Kimmel-Gerke Associates, Ltd.,
St. Paul, MN. 55108, (612) 330-3728.
2.
Walt Jung, Dick Marsh, Picking Capacitors, Parts 1 & 2, Audio,
February, March, 1980.
3.
Tantalum Electrolytic and Ceramic Capacitor Families, Kemet
Electronics, Box 5928, Greenville, SC, 29606, (803) 963-6300.
4.
Type HFQ Aluminum Electrolytic Capacitor and type V Stacked
Polyester Film Capacitor, Panasonic, 2 Panasonic Way, Secaucus,
NJ, 07094, (201) 348-7000.
5.
OS-CON Aluminum Electrolytic Capacitor 93/94 Technical Book,
Sanyo, 3333 Sanyo Road, Forrest City, AK, 72335, (501) 633-6634.
6.
Ian Clelland, Metalized Polyester Film Capacitor Fills High Frequency
Switcher Needs, PCIM, June 1992.
7.
Type 5MC Metallized Polycarbonate Capacitor, Electronic Concepts, Inc.,
Box 1278, Eatontown, NJ, 07724, (908) 542-7880.
8.
Walt Jung, Regulators for High-Performance Audio, Parts 1 and 2,
The Audio Amateur, issues 1 and 2, 1995.
9.
Henry Ott, Noise Reduction Techniques in Electronic Systems,
2d Ed., 1988, Wiley.
10.
Fair-Rite Linear Ferrites Catalog, Fair-Rite Products, Box J, Wallkill,
NY, 12886, (914) 895-2055, http://www.fair-rite.com.
11.
Type EXCEL leaded ferrite bead EMI filter, and type EXC L leadless
ferrite bead, Panasonic, 2 Panasonic Way, Secaucus, NJ, 07094,
(201) 348-7000.
12.
Steve Hageman, Use Ferrite Bead Models to Analyze EMI Suppression,
The Design Center Source, MicroSim Newsletter, January, 1995.
13.
Type 5250 and 6000-101K chokes, J. W. Miller, 306 E. Alondra Blvd.,
Gardena, CA, 90247, (310) 515-1720.
14.
DIGI-KEY, PO Box 677, Thief River Falls, MN, 56701-0677,
(800) 344-4539.
10.56
HARDWARE DESIGN TECHNIQUES
15.
Tantalum Electrolytic Capacitor SPICE Models, Kemet Electronics,
Box 5928, Greenville, SC, 29606, (803) 963-6300.
16.
Eichhoff Electronics, Inc., 205 Hallene Road, Warwick, RI., 02886,
(401) 738-1440, http://www.eichhoff.com.
17.
Practical Design Techniques for Power and Thermal Management,
Analog Devices, 1998, Chapter 8.
10.57
HARDWARE DESIGN TECHNIQUES
DEALING WITH HIGH SPEED LOGIC
Much has been written about terminating printed circuit board traces in their
characteristic impedance to avoid reflections. A good rule-of-thumb to determine
when this is necessary is as follows: Terminate the line in its characteristic
impedance when the one-way propagation delay of the PCB track is equal to or
greater than one-half the applied signal rise/fall time (whichever edge is faster). A
conservative approach is to use a 2 inch (PCB track length)/nanosecond (rise-, falltime) criterion. For example, PCB tracks for high-speed logic with rise/fall time of
1ns should be terminated in their characteristic impedance if the track length is
equal to or greater than 2 inches (including any meanders). Figure 10.48 shows the
typical rise/fall times of several logic families including the SHARC DSPs operating
on +3.3V supplies. As would be expected, the rise/fall times are a function of load
capacitance.
TYPICAL DSP OUTPUT RISE TIMES AND FALL TIMES
■ GaAs: 0.1ns
■ ECL: 0.75ns
■ ADI SHARC DSPs: 0.5 ns to 1 ns (Operating on +3.3V Supply)
ADSP-21060L
SHARC:
Figure 10.48
This same 2 inch/nanosecond rule of thumb should be used with analog circuits in
determining the need for transmission line techniques. For instance, if an amplifier
must output a maximum frequency of fmax, then the equivalent risetime, tr, can be
calculated using the equation tr = 0.35/fmax. The maximum PCB track length is
then calculated by multiplying the risetime by 2 inch/nanosecond. For example, a
maximum output frequency of 100MHz corresponds to a risetime of 3.5ns, and a
track carrying this signal greater than 7 inches should be treated as a transmission
line.
10.58
HARDWARE DESIGN TECHNIQUES
Equation 10.1 can be used to determine the characteristic impedance of a PCB track
separated from a power/ground plane by the board’s dielectric (microstrip
transmission line):
Z o (Ω) =
 5.98d 
ln 

 0.89w + t 
ε r + 1.41
87
Eq. 10.1
where εr = dielectric constant of printed circuit board material;
d = thickness of the board between metal layers, in mils;
w = width of metal trace, in mils; and
t = thickness of metal trace, in mils.
The one-way transit time for a single metal trace over a power/ground plane can be
determined from Eq.10.2:
t pd (ns / ft) = 1.017 0.475ε r + 0.67
Eq. 10.2
For example, a standard 4-layer PCB board might use 8-mil wide, 1 ounce (1.4 mils)
copper traces separated by 0.021" FR-4 (εr=4.7) dielectric material. The
characteristic impedance and one-way transit time of such a signal trace would be
88Ω and 1.7ns/ft (7"/ns), respectively.
The best ways to keep sensitive analog circuits from being affected by fast logic are
to physically separate the two and to use no faster logic family than is dictated by
system requirements. In some cases, this may require the use of several logic
families in a system. An alternative is to use series resistance or ferrite beads to
slow down the logic transitions where the speed is not required. Figure 10.49 shows
two methods. In the first, the series resistance and the input capacitance of the gate
form a lowpass filter. Typical CMOS input capacitance is 5pF to10pF. Locate the
series resistor close to the driving gate. The resistor minimizes transient currents
and may eliminate the necessity of using transmission line techniques. The value of
the resistor should be chosen such that the rise and fall times at the receiving gate
are fast enough to meet system requirement, but no faster. Also, make sure that the
resistor is not so large that the logic levels at the receiver are out of specification
because of the voltage drop caused by the source and sink current which flow
through the resistor. The second method is suitable for longer distances (>2 inches),
where additional capacitance is added to slow down the edge speed. Notice that
either one of these techniques increases delay and increases the rise/fall time of the
original signal. This must be considered with respect to the overall timing budget,
and the additional delay may not be acceptable.
Figure 10.50 shows a situation where several DSPs must connect to a single point,
as would be the case when using read or write strobes bidirectionally connected
from several DSPs. Small damping resistors shown in Figure 10.50A can minimize
ringing provided the length of separation is less than about 2 inches. This method
will also increase rise/fall times and propagation delay. If two groups of processors
must be connected, a single resistor between the pairs of processors as shown in
Figure 10.50B can serve to damp out ringing.
10.59
HARDWARE DESIGN TECHNIQUES
DAMPING RESISTORS SLOW DOWN FAST
LOGIC EDGES TO MINIMIZE EMI/RFI PROBLEMS
R
LOGIC
GATE
< 2 inches
LOGIC
GATE
CIN
Risetime = 2.2 R·CIN
> 2 inches
R
LOGIC
GATE
LOGIC
GATE
C
CIN
Risetime = 2.2 R·(C + CIN)
Figure 10.49
SERIES DAMPING RESISTORS
FOR SHARC DSP INTERCONNECTIONS
A
STAR CONNECTION
DAMPING RESISTORS
SHARC
DSP
USE FOR RD, WR
STROBES
SHARC
DSP
<2"
10Ω
Ω
EACH
SHARC
DSP
SHARC
DSP
NOTE: THESE TECHNIQUES
INCREASE RISE/FALL TIMES
AND PROPAGATION DELAY
B
SINGLE DAMPLING
RESISTOR BETWEEN
PROCESSOR GROUPS
SHARC
DSP
SHARC
DSP
20Ω
Ω
SHARC
DSP
Figure 10.50
10.60
<2"
SHARC
DSP
HARDWARE DESIGN TECHNIQUES
The only way to preserve 1ns or less rise/fall times over distances greater than
about 2 inches without ringing is to use transmission line techniques. Figure 10.51
shows two popular methods of termination: end termination, and source
termination. The end termination method (Figure 10.51A) terminates the cable at
its terminating point in the characteristic impedance of the microstrip transmission
line. Although higher impedances can be used, 50Ω is popular because it minimizes
the effects of the termination impedance mismatch due to the input capacitance of
the terminating gate (usually 5pF to 10pF). In Figure 10.51A, the cable is
terminated in a Thevenin impedance of 50Ω terminated to +1.4V (the midpoint of
the input logic threshold of 0.8V and 2.0V). This requires two resistors (91Ω and
120Ω), which add about 50mW to the total quiescent power dissipation to the
circuit. Figure 10.51A also shows the resistor values for terminating with a +5V
supply (68Ω and 180Ω). Note that 3.3V logic is much more desirable in line driver
applications because of its symmetrical voltage swing, faster speed, and lower
power. Drivers are available with less than 0.5ns time skew, source and sink
current capability greater than 25mA, and rise/fall times of about 1ns. Switching
noise generated by 3.3V logic is generally less than 5V logic because of the reduced
signal swings and lower transient currents.
TERMINATION TECHNIQUES FOR CONTROLLED
IMPEDANCE MICROSTRIP TRANSMISSION LINES
TYPICAL DRIVERS:
■ 74FCT3807/A (IDT)
■ 74ACTQ240 (Fairchild)
A
ZO = 50Ω
Ω
+1.4V
+3.3V
+5.0V
120Ω
Ω
30mW
180Ω
Ω
72mW
+1.4V
91Ω
Ω
22mW
GROUND PLANE
B
68Ω
Ω
29mW
END TERMINATION
ZO ≈ 10Ω
Ω
SOURCE TERMINATION
39Ω
Ω
RULE OF THUMB:
ZO = 50Ω
Ω
USE TRANSMISSION LINE IF DISTANCE IS
MORE THAN 2"/ns OF LOGIC RISE/FALL TIME
50Ω
Ω PC BOARD TRANSMISSION LINE DELAY ≈ 1ns / 7"
Figure 10.51
The source termination method, shown in Figure 10.51B, absorbs the reflected
waveform with an impedance equal to that of the transmission line. This requires
about 39Ω in series with the internal output impedance of the driver, which is
generally about 10Ω. This technique requires that the end of the transmission line
be terminated in an open circuit, therefore no additional fanout is allowed. The
10.61
HARDWARE DESIGN TECHNIQUES
source termination method adds no additional quiescent power dissipation to the
circuit.
Figure 10.52 shows a method for distributing a high speed clock to several devices.
The problem with this approach is that there is a small amount of time skew
between the clocks because of the propagation delay of the microstrip line
(approximately 1ns /7"). This time skew may be critical in some applications. It is
important to keep the stub length to each device less than 0.5" in order to prevent
mismatchs along the transmission line.
CLOCK DISTRIBUTION USING
END-OF-LINE TERMINATION
+3.3V
TRANSMISSION LINE ZO = 50Ω
Ω
CLOCK
+1.4V
120Ω
Ω
30mW
91Ω
Ω
22mW
SHARC
DSP
SHARC
DSP
SHARC
DSP
50Ω
Ω PC BOARD TRANSMISSION LINE DELAY ≈ 1ns / 7"
NOTE: KEEP STUB LENGTH < 0.5"
NOT RECOMMENDED FOR SYNCHRONIZED SHARC OPERATION!
Figure 10.52
The clock distribution method shown in Figure 10.53 minimizes the clock skew to
the receiving devices by using source terminations and making certain the length of
each microstrip line is equal. There is no extra quiescent power dissipation as would
be the case using end termination resistors.
Figure 10.54 shows how source terminations can be used in bi-directional link port
transmissions between SHARC DSPs. The output impedance of the SHARC driver
is approximately 17Ω, and therefore a 33Ω series is required on each end of the
transmission line for proper source termination.
The method shown in Figure 10.55 can be used for bi-directional transmission of
signals from several sources over a relatively long transmission line. In this case,
the line is terminated at both ends, resulting in a DC load impedance of 25Ω.
SHARC drivers are capable of driving this load to valid logic levels.
10.62
HARDWARE DESIGN TECHNIQUES
PREFERRED METHOD OF CLOCK DISTRIBUTION
USING SOURCE TERMINATED TRANSMISSION LINES
ZO ≈ 10Ω
Ω
*
39Ω
Ω
> 4"
ZO = 50Ω
Ω
SHARC
DSP
ZO = 50Ω
Ω
SHARC
DSP
ZO = 50Ω
Ω
SHARC
DSP
ZO ≈ 10Ω
Ω
CLOCK
*
39Ω
Ω
ZO ≈ 10Ω
Ω
* Same
Package
*
39Ω
Ω
Figure 10.53
SOURCE TERMINATION FOR BI-DIRECTIONAL
TRANSMISSION BETWEEN SHARC DSPs
ADSP-2106x
ADSP-2106x
OFF
LENGTH > 6"
33Ω
Ω
ZO = 50Ω
Ω
33Ω
Ω
ON
Ω
ZO ≈ 17Ω
LINK PORT
TRANSMITTER
LINK PORT
RECEIVER
Figure 10.54
10.63
HARDWARE DESIGN TECHNIQUES
SINGLE TRANSMISSION LINE
TERMINATED AT BOTH ENDS
SHARC
DSP
SHARC
DSP
SHARC
DSP
+3.3V
+3.3V
120Ω
Ω
30mW
Ω
ZO = 50Ω
LENGTH > 10"
+1.4V
91Ω
Ω
22mW
120Ω
Ω
30mW
91Ω
Ω
22mW
SHARC
DSP
SHARC
DSP
SHARC
DSP
NOTE: KEEP STUB LENGTH < 0.5"
NOT RECOMMENDED FOR CLOCKS IN SYNCHRONIZED SHARC OPERATION!
Figure 10.55
10.64
HARDWARE DESIGN TECHNIQUES
REFERENCES ON DEALING WITH HIGH SPEED
LOGIC:
1.
Howard W. Johnson and Martin Graham, High-Speed Digital Design,
PTR Prentice Hall, 1993.
2.
EDN’s Designer’s Guide to Electromagnetic Compatibility, EDN,
January, 20, 1994, material reprinted by permission of Cahners Publishing
Company, 1995.
3.
Designing for EMC (Workshop Notes), Kimmel Gerke Associates, Ltd., 1994.
4.
Mark Montrose, EMC and the Printed Circuit Board, IEEE Press,
1999 (IEEE Order Number PC5756).
10.65
HARDWARE DESIGN TECHNIQUES
10.66